"A potential audience of millions of users" means less than it implies. If I just put my work online, have I got the audience of billions of Internet users? Probably yes, but it doesn't mean much - I still need to tell people about my work somehow so they would be interested in it.
If we think about how App Store can actually actively help us in promoting an app, without specifically buying ad in the App Store - which doesn't cost $99/year, it costs much more - it's not exactly appealing. Epic's CEO showed how bad the App Store search is in finding apps that users specifically search for: https://twitter.com/timsweeneyepic/status/101985252700794470...
And why wouldn't it be bad? Iphone users don't have any better.
They are not subsidized at all.
I've seen cheaper subscription services that also come with support, hosting and services.