I suspect that's intentional; after all many people want to listen to music while doing something else, so being able to free that screen real estate while continuing to play the song is beneficial. Personally, playing music was one of my main Android Firefox + YouTube use cases for a while.

I think you should be more confident! I would have ✓:ed ("agreed") if not for how it could be taken as excessively crediting your "(perhaps wrongly)" as truth-tracking.

Also, off the top of my head, I think there should be a "Be more confident!" reaction.

I think there's something that might be perceived as a quality decline, a decrease in how alive the text feels, as Wikipedia follows its strict notability policy and, as a result, gets more and more academic and dry as the more creative/informal/adventurous writers cannot write the kind of texts they want, and therefore move to smaller, more informal, wikis, where on-topic-ness is the criterion of inclusion rather than notability, and it's applied rather loosly.

Nobody in their right mind would claim that that is an important problem

I would