That's the only reason why I've switched to Jellyfin. It's not better than plex. It's definitely worse, but it's open, and that's how I am sure it will work forever.
Why are you so sure about it? IIRC Emby was also open sourced before it wasn't, at which point it was forked into Jellyfin. Based on that alone, I wouldn't be too surprised if Jellyfin pulls the same thing down the road, forcing you to move to yet another fork down the line.
Who's going to sue the rest of the developers who keep working on the open version we'd rather use?
But that doesn't guarantee they wouldn't if Jellyfin approached them like "here's $X for writing this code". Or try hiring them from the start and asking them to sign a contract that transfers their copyright over to the company. Or maybe the contributors would tell them no, make a media fuss about it, and then Jellyfin's team could track down their commits and just re-write them.
Of course, I'm not saying that this will happen, nor I am saying that the Jellyfin's team will even make an attempt at going down this road, but the fact that it's open sourced at the moment is not a guarantee that it wouldn't be in the future.
Emby had a fair amount of contributors as well (https://github.com/MediaBrowser/Emby/graphs/contributors) and then they've just stopped pushing more commits to it. As far as I'm aware, none of those people sued them.
Sure you can find and rewrite that person's contributions, but that person can also fork again.
That's exactly OP's point, whereas I can't just switch to a fork of Plex.
If jellyfin ever becomes closed, people can continue the project from the last open release.
And we will keep moving to other oss forks until they realize that closing the source down will mean that they will never get our business.
There are better ways to make money than bait-and-switching with the licensing. It's not my responsibility to keep them working if what they are offering goes against what I value as a consumer.