I think you're getting confused here with app limitations vs platform limitations. YouTube Music not streaming without the paired phone on the same wifi is an app limitation: other music apps like Spotify work without it. Lacking a watch app (or having a bad watch app) probably never lost any non fitness company any customers. A good iPhone app is much more make or break.

So, when Apple released the iPhone, they went out of their way to make sure that YouTube and Google Maps were on the device, to the point of making special deals with Google and developing all of the software for them (as they didn't want to give up any control)... I maintain that, given how they already have a partnership with OpenAI, it is a strange lack of vision that I can't go down to the beach and spend my day talking to ChatGPT to get things done.