As randomdata pointed out, I think this goes more to proving my point than yours. They did a lot of work to get to that point. Contrast that with "Tiny Wings" that showed up and started making a huge amount of money.

I'll grant you that it's more effort today than it was with an App, but an App platform provides an easier method of discovery [1], an easy method for billing and even an advertising platform. With the web this is all on you and the web is more ad focused so you're probably more likely to be competing with free.

[1] Random chance with the user entering the right google keywords vs doing a category search in the app store and knowing you can see everything in that category.

Not a good example. 37signals had a huge following long before Basecamp. Their success in web apps was largely from leveraging that existing fan base. Most developers do not have the luxury.