No non-free upgrades. To release version 2 of your app with a fee you have to release a new app. There's no connection between the two and AFAIK no way to provide a discount for existing users.
As for the MAS harming devs it's the "official store of OSX" so users are arguably being trained to look there and only there for apps. So sure you can look elsewhere but Apple is fighting against you.
30% cut is very high as well. I think existing payment systems for shareware apps were 5% or less. What do you get for your 30%? Support? No. Discovery? Not really. File Hosting? Ok, maybe but is that worth 30%? Easy upgrade and install. That's about it but usually that would be mostly handled by a free library.
The other things, yes, but 30% cut isn't a real problem. Have some perspective. You used to pay a lot more than that for packaging and distribution as a software developer, when software was still physical goods. (Yes, shareware, sure, but the number of developers who made a living out of shareware was extremely small.)
Have some history. After the physical distribution are and before MAS, there was (and still is) a large number of indie OS X developers distributing digitally. For a long time, FastSpring was the de-facto standard and its rate is 8.9%. Add a CDN or DevMate and 10% seems reasonable. DevMate in particular makes the difference stark: more flexibility, more functionality, 20% less.
The difference is history and options. Computers have a 30 year history of apps and buying options. Smartphones don't. iOS has only one way to get an app on the phone. Computers don't
As for the MAS harming devs it's the "official store of OSX" so users are arguably being trained to look there and only there for apps. So sure you can look elsewhere but Apple is fighting against you.
30% cut is very high as well. I think existing payment systems for shareware apps were 5% or less. What do you get for your 30%? Support? No. Discovery? Not really. File Hosting? Ok, maybe but is that worth 30%? Easy upgrade and install. That's about it but usually that would be mostly handled by a free library.