I run a lot of self-hosted software services, many of which have their own internal account system and auth. None of the features you mentioned require 3rd party cloud based auth.

I did pay for Plex prior to the cloud auth change, so for me it's a bait and switch, but my concerns are much more about privacy.

One day Plex will be bought by a large media company, and my (and my kids') viewing data and library catalogue data will be owned by MGM, Disney, Fox, etc...