I'm curious how you would define "polish" here: do you mean spending time on fixing current features as opposed to adding new ones? If so, I can see the argument there: a lot of software tends to prioritize new features over long-term stability.
On the other hand, I've had a fairly good experience with Jellyfin -- it's mostly been a set-and-forget solution.
Sure, by "polish" I mean to get it somewhat close to parity with Plex. If it's great as is, perhaps no additional polish is required. I prefer to invest in open source tools that will remain open, whenever possible. It is a component of defending against enshittification, because that which has been built cannot then be revoked or gated.
At the end of the day, it’s always about control.
On the other hand, I've had a fairly good experience with Jellyfin -- it's mostly been a set-and-forget solution.