Honestly for the situation you presented you'll find very few QOL improvements by upgrading. You could probably sit on 1.5 forever on that system (internal jenkins) forever.
The biggest driver is actually just to not be behind.
You can tell already from what little conversation we've had that "always be upgrading" is not a cultural practice here (yet.)
We have regular meetings about changing that! Had two just yesterday. Chuckle
It just hasn't been a priority. I have no need for RBAC at this point, as I am the only cluster admin, and the whole network is fairly well isolated.
I couldn't really think of a good reason to not upgrade when it came time to kubeadm init again, but then I realized I could probably save ten minutes by not upgrading, it was down, and I didn't know what the immediate consequences of adding RBAC would be for my existing Jenkins deployment and jobs.
Chances are it would have worked.