From there it's not a huge jump to allow flatpaks that are closed source and/or not written with GTK, certainly more realistic than with non-sandboxed DEBs.
But as a former elementary user and micro-contributor, I also think that the App Store model has been a disaster for elementary OS. It means that developers who are perfectly capable of fixing the buggy stock apps are incentivized to instead start their own app and (fail to, presumably) get paid for it. Random example from a very prolific developer in the golden age of eOS 5:
https://appcenter.elementary.io/com.github.artemanufrij.play...
Now this app has been abandoned, and instead of taking over this developer's personal app, the next person who wants to write a music player will start another one. And so on.