I was worried about this too, but have you tried it lately after doing an import of your Play Music data? I've been a subscriber to Play Music nearly since its inception and none of the functionality I cared about seems to be missing from YT Music in its current form. Indeed there are a couple of YT Music features I am using occasionally (the lyrics for example).

I think the impetus here is really just to unify all of their streaming content services under the YT brand, which if anything instills some confidence in me that they won't simply kill it off (Play Music was feeling like abandonware for a long time, being under the YT umbrella with an app under active development seems like an improvement).

That said, there are plenty of other ways to get the actual music files. I treat this service as a way to discover playlists and new artists only, and if there's something I want to retain access to I acquire my own copy to ensure I will not lose access to it in the future.

I tend to attribute the product "churn" due to how promotions/raises are handled where releasing new products is heavily favored over maintaining existing ones.