Android has been friendly toward removing the mothership since day one. Anyone can deliver APKs using either a homegrown app store (see Amazon's App Store app) or by just letting the user download APKs from the browser.
The problem is that most app developers currently -only- distribute their app via Google Play, as it's the primary distribution channel that devices tend to come with. This isn't a complete lock-in, since you can still get those installed APKs from a device where they've been installed and archive them or reinstall them on other devices. (Refer to ApkMirror as an ethically dubious prime example of this)
Still, this is nothing to hold against Android itself.
Do you know why they're planning to drop Firefox? And why would Chromium never make it on there? I don't know much about F-Droid and its limitations.
EDIT: Okay I did the Google search that I should've just done in the first place. Apparently they're not dropping Firefox fully, but rather forking it to "remove the proprietary binaries out of the official builds" (Google Play API, etc). I suppose APIs like that are important enough to your use-case that you need an alternative source for Firefox?
This is presumably why Firefox's removal has been continuously postponed. Reliability is one of the reasons why I would prefer first-party Firefox over a third-party fork.
Though there are good reasons why F-Droid doesn't include Chromium and seeks to remove Firefox, the absence of many open source Android apps from F-Droid's catalog has a direct impact on utility. It's also not uncommon for apps on F-Droid to lag behind the version on Google Play or the latest release direct from the developer. (Again, there are good reasons why they screen apps, but it does impose a delay.)
I appreciate F-Droid's screening process, and I wouldn't want it to go away, but it'd be nice to have an alternative open source Android app catalog that's a little more liberal.
Gotcha, so very roughly speaking the same reason that people sometimes leave Debian for Ubuntu, because they hit the balance between ideological purity and convenience a bit better.
And even if after much begging you get a .apk from them, it often still won't work on a Google-free Android because it uses the damn Google API that requires the Google binaries that you don't want to soil your system with.
The problem is that most app developers currently -only- distribute their app via Google Play, as it's the primary distribution channel that devices tend to come with. This isn't a complete lock-in, since you can still get those installed APKs from a device where they've been installed and archive them or reinstall them on other devices. (Refer to ApkMirror as an ethically dubious prime example of this)
Still, this is nothing to hold against Android itself.