Well, tough. As an app developer, I don't bother with users who don't use Play. I prefer worrying about Play versions instead of Android versions. There are a lot of useful features which we would otherwise wait for new OS releases to be able to use.

The vast majority of which have no reason to be tied to Play, they're just (ab)using Play to auto-install and auto-update it everywhere. An effective way around carrier update delays, to be sure, but far from the only option.

It could've been an app. Or more than one. And then there could've been competition. Instead it's baked into a monster install that does who-knows-what when it invisibly updates itself.

That's fair criticism, but "we" are likely to complain that "Google is installing many apps, and I don't know what they're doing". What also happens when I install "Core" v11.0.4, and then "Cast" doesn't install? Or the user stumbles upon "PubSub", and removes updates?

I'm inclined to think this is an all or nothing here, but since we don't have access to the source, I won't know how Google has implemented Play Services.

Can you kindly give an example of which APIs shouldn't be tied to Play though? I've never thought about it.

Does Chromecast really require a Play login? On iOS you can cast with no login at all

I don't believe it requires a login, but it does require Play services, which requires giving root to Google via a closed-source binary.