"not because the airlines have figured out some advantage in point to point routes."
Southwest has figured out the advantage. People want point to point and the profits and continued expansion of Southwest will hopefully clue the other airlines onto that fact. I fear the day when Southwest no longer has any competition and decides to convert over to a hub system to save on costs. Using current accounting methods, airlines see huge advantages in using a hub system. If they had to charge all the wasted time the hub system costs its passengers, they might see why Southwest keeps getting bigger and bigger. When looking to fly, I always sort for non-stops only and consider that as the price to fly to my destination (luckily I live in the Bay Area that has airport competition). Only when no non-stop flights exist, do I consider multiple stop flights.
As I said, that is only true regionally or domestically. Even then, Southwest has minihubs in order to connect the routes that don't have enough volume to justify direct flights. You can see this yourself: if you track their flight numbers, each "flight" is typically 3 to 4 legs, at least one of which is a southwest hub. For example, Seattle's only point to point route on Southwest is to San Jose where Seattle has a lot of cultural and business connections due to the tech industry. Anywhere else from Seattle is either direct because Southwest happens to operate a hub there, or it transfers through a hub.
Unique to southwest (I'd say it's a strategic advantage, but only viable regionally) is the use of focus cities like Sacramento. These operate essentially as hubs, but aren't called hubs because they don't have maintenance operations there. Southwest's route planning model within regions is quite unique, and you can see how it works by tracking a flight number. Almost all of their regional flights are 3 leg flights: (Spoke -> Hub -> Focus City -> Spoke) or (Spoke -> Focus City -> Hub -> Spoke). This allows them to have lots of hubs (which allows for a lot of direct flights), while still allowing nearly all of their fleet to have access to a hub's maintenance operations, which is probably the most expensive part of operating a hub. This gives the popular illusion of having an extensive point to point network, but in reality nearly all flights connect to a hub or pseudo hub.
There is absolutely no doubt that people prefer direct flights. Airlines running hubs have basically resigned themselves to the fact that if southwest opens a direct route between any of their non-hub cities, they've lost almost all of their non-frequent-flier customers on that specific city pairing to Southwest. That means that direct flights only make sense if there is enough volume on the specific corridor, and that only ever happens regionally, because that is where all the cultural and business connections are.
So a city is a hub when the airline does maintenance at that location? How often does an airplane require maintenance? Then you talk about minihubs, pseudo hubs, and Focus Cities. What does point to point mean for someone in the business? Seems to me with all of this contorted vocab to keep Southwest in the hub system it might mean it really is a different type of airline. Not hub based but something else (as you say it is not a point to point airline)?
Maybe the word I should use is non-stop for the type of flight that people want. "point to point" seems to have some technical meaning that I don't grok. What makes the Seattle to San Jose a point to point route but other Seattle flights by Southwest not point to point?
Airlines need small amounts of maintenance daily, but occasionally intense maintenance on short notice. When you have to use 3rd party maintenance facilities, it is extremely expensive. Most airlines run their own maintenance facilities at hubs because most of their planes pass through hubs, and a huge expense with maintaining a hub is running a maintenance operation.
Southwest's strategy maintains focus cities (which I've used interchangably with minihub or pseudo hub), which operates as a hub from the passenger's perspective, but does not include maintenance facilities. This works out well for them because they operate short haul multi-leg flights on routes that ensure they always have a maintenance facility available to them once per day. So a three leg flight will start at a spoke city, have one intermediate stop at an full hub with maintenance operations, and one stop at a minihub or focus city, before ending at a spoke city. By only having maintenance facilities at half their "hubs", they can have many hubs at a fraction of the cost.
Once you get out of the regional business, those multi-leg flight paths no longer make as much sense, because spoke->hub->hub->spoke flight times with a return trip will exceed pilot's allowable operating hours, requiring you to pay pilots to live in two locations instead of one. Southwest has ripped up the regional market with this strategy, but it absolutely doesn't work for anything outside of the regional market.
What fraction of cities in the US that Southwest services would you call a hub or minihub? Or the fraction of flights that go to spokes? By playing around with this map[1] I would say that southwest has 90-95% of flights going hub to (mini)hub. While delta is very different[2]. Looks like the hub model lets an airline service many smaller cities while the Southwest model services only larger cities but makes most of them a (mini)hub. If I live in North Dakota and can afford to fly, I sure would want the hub system to exist, but for a large number of mid sized cities the Southwest model works much better.
You can see their full hubs and focus cities on their wikipedia page. And it isn't necessarily due to being large or small. Some of their hubs are in large cities and some are in small cities. My metro area is quite large (Seattle), but it is only a spoke city, not a hub or a minihub.
Operating bases (full hub)
Atlanta
Baltimore
Chicago–Midway
Dallas–Love
Denver
Houston–Hobby
Las Vegas
Oakland
Orlando
Phoenix–Sky Harbor
Focus cities (mini hub)
Austin
Fort Lauderdale
Los Angeles
Nashville
Sacramento
San Diego
San José (CA)
St. Louis
Tampa
Arguing about the definition of words can be sort of silly but I guess the last thing I'll say here is that, from a travelers perspective, Southwest, with its 19 hubs/minihubs in the US and scheduling philosophy, many cities that are not hubs look like hubs to the traveler. Take ABQ. A non-hub city for Southwest and Delta. In Southwest's system ABQ connects directly with 15 other cities. For Delta only 4(this could be a cherry pick, but it was the first one I looked at). Southwest focuses on the middle class and small business owners (no first class seating, no expensive flights to Aspen, Jackson Hole, or Sun Valley, no meals, everyone can check bags for free without being on a frequent flyer list, no fees for changing flights, etc) while all the others seem to look to make money on the first class seats and fill the rest as an after thought.
Being in the first group, I like Southwest's system better and wish another airline could try the same system for competition. Jet Blue seemed to start out that way but I have not flown them in years. A quick check of their site it seems that have baggage fees and change fees and look pretty hub centered.
There's two components that make an airport into a hub:
1) Transfer operations. This means flight schedules are almost invariably determined by the hub airport, because they time arrivals and departures to happen in waves, which minimize the amount of time that passengers have to wait for connections. This means that you'll have a lot of staff to handle the peak load of a transfer wave, while that staff might be relatively idle at other times.
2) Maintenance operations: this means doing routine maintenance during layover, as well as swapping out planes for intense maintenance. This is why your hub layover doesn't have a pre-determined terminal for your second leg flight...it can and regularly will change if a plane needs to be swapped out for maintenance.
A spoke city will have neither transfer operations nor maintenance operations. A minihub might have transfer operations but not maintenance operations (I've explained these are only viable regionally in other comments). But a full hub will have both.
Southwest has figured out the advantage. People want point to point and the profits and continued expansion of Southwest will hopefully clue the other airlines onto that fact. I fear the day when Southwest no longer has any competition and decides to convert over to a hub system to save on costs. Using current accounting methods, airlines see huge advantages in using a hub system. If they had to charge all the wasted time the hub system costs its passengers, they might see why Southwest keeps getting bigger and bigger. When looking to fly, I always sort for non-stops only and consider that as the price to fly to my destination (luckily I live in the Bay Area that has airport competition). Only when no non-stop flights exist, do I consider multiple stop flights.