> I’m basically presenting adapting to a new UI as adapting to the faster page load.
But I as a user don't have to do anything to adapt to a faster page load. Probably I won't even notice it. If you change the UI out from under me--even if it actually improves it--then I have to learn your new clever UI and that breaks my focus. It imposes a cost on me that I did not consent to. Instead of getting work done, I have to learn your new UI. So these things are completely different.
EDIT: To phrase it another way, what is the justification for your UI "improvement"? I claim the reason you have to make an improvement at all is that you released your product before it was finished. You shouldn't do that. If I'm paying for your product I'm not your "beta tester" so don't treat me like one.
EDIT2: Another way to look at it: your UX is your public API. Once you release it, that's final.
> perhaps some plugin manufacturer now needs to do some extra work
As a user, I couldn't care less how much work needs to be done, and who needs to do it. I just want the tool to work the same day in and day out.