Something that's getting glossed over in these comments is the ability to easily watch or listen to friends' media.
I have my own library with about 1k movies, a bunch of anime and TV, and 10k albums. But I have like 6 or 7 friends with libraries even larger. My one friend has 37k albums, they all have thousands of movies I never even heard of, etc. It really makes it like my own mini streaming service, and I love throwing on a huge music library on shuffle via plexamp while driving to/from work.
I paid like $70 for a lifetime pass years ago, so I'm along for the ride I guess. I really rely on the music aspect of it, I haven't had a spotify subscription in like 7 years.
I know they changed a lot lately, and particularly what pisses me off is how vague and how they intentionally obfuscate how their model works now. I have friends that for years used my library, and recently have been like "I saw Plex started charging now so I stopped using it" and I have to be like "no it's still free because I have a lifetime pass". It's definitely just to trick people into getting monthly subscriptions.