This is why I only buy disks and rip them. My Plex server will never close down or tell me I can't watch a movie anymore.

>My Plex server will never close

Plex is proprietary and can shut down any time. Although it runs locally, it contacts the plex servers for quite a lot of stuff. Granted, you won't lose your media and can replace plex easily.

any good alternative? Kodi? Universal Media Server?

Emby works well for me, better than Plex ever did. https://emby.media/

Emby became proprietary as well. So no real reason to choose it.