Why so many people went for it when there were better open-source alternatives available...
I like how you phrase this like Jellyfin has been around as long as Plex, and is as mature in its development. For many people, Jellyfin wasn't really "there" until version 10.9 -- and that was less than two years ago. Many people had Plex servers up and long established before then, and see no reason to change after investing lots of time in their existing setup. The main complaints driving people to Jellyfin now are:
1) The addition of advertising-supported streaming content.
2) The recent push to make the ecosystem into some lame social media network revolving around TV/movies.
3) The most recent changes to pricing and remote access no longer being free.
- You can disable the first one at the account-level very easily.
- You can also hide/disable the effects of the second and set privacy settings to tamp it down.
- And for anyone who already had a Lifetime Plex Pass, the third is a non-issue.