That's not a sunk cost fallacy at all. I'm not playing catch up, I'm already ahead.
I am thinking about it logically. Plex is the superior experience, and considering I've already gotten my money's worth out of it, they are both free. So why would I use the service that works less well for my use case?
If the user experience continues to degrade, when it finally gets to jellyfin level, I'll switch.
Perfectly logical, no fallacies here.