There's no reason to use Plex as a front end for all other streaming services.
I’ll disagree here. If I don’t know where something is, I can just list all of the streaming services I subscribe to in Plex and just go there to start watching something.
Moreover the individual streaming services have designed their apps to literally hide the shows you’ve been watching whereas Plex keeps things front and centre. Much smoother.
To help them out, you could unselect those services which require payment inside Plex, and just leave the streamers to which they have access?
I’m not aware of any alternative services on Apple TV that do this. Certainly Just Watch and services that interact with it will work, but not with the same integration with my own library.