Changed aesthetics are fine. Even when they suck you eventually get used to them. Changed functionality - which with UI refreshes often entails removal of more advanced or subtle features - is the killer. Designers who don't understand how products work in depth look at complexity they don't understand and decide nobody needs it if they personally don't get what it's for.
It is a problem even when it comes to aesthetics. Turning up the whitespace 300% makes things worse, it's not even just a subjective design choice.
The problem is precisely what you say: UI """designers""" don't make evidence-based decisons and don't do research or look for user feedback, they simply go with whatever they subjectively think best.
