Why stop there? Web apps can trivially be taken down if your domain registrar, hosting provider, datacenter partner or payment solution decides to kick you out.
You have alternatives for those things though, in other countries if necessary. Ok, ok, accepting payments is hard -- if you're getting booted from your processors because, say, you're in a controversial industry, you might have to get creative (look at the US cannabis industry for such creativity).
There many hosting provider, datacenter, payment gatway to choose from. But just 2 appstore to use (technically android can have as many as you want, but how many pp use 3rd store right now?)
The problem I have with third party app stores is you have to open up permissions to install any third-party app -- you can't just say "Trust apps from the following app stores" (at least that I've found, although it has been a while since the last time I looked into it).
compare to china where google is blocked, which as a result has dozens of competing appstores. each phone brand has their own, and then some.
i do believe that if google didn't have an appstore that it forces android licensees to use, then we would have a similar variety of appstores world wide.