The reality is, people hate change period. The flat UI that everyone is now pining for was pretty roundly hated when it debuted too. And the reaction to the original OS X interfaces with its "lickable" buttons was also pretty full of hate and anger. And in every one of those cases, some of the complaints were valid, and Apple in some of those cases walked the changes back or adjusted them. But a large UI change also just comes with a lot of hate in general because everything is different.
There are plenty of parts about the new liquid glass UIs that are fine. They may not be your preferred aesthetic choice, but the flat UI or the heavy skeumorphic UI of days of old wasn't everyone's preferred aesthetic choice either. Personally I've found myself recently looking for KDE themes that bring back late 90's platinum/beOS/Next style "drawn 3D" UIs (which is a terrible term for it, but I don't have a better word at hand for what I'm thinking of). Liquid Glass will be refined, improved and sanded down into something people are fine with and in 10 more years when Apple releases a new UI we'll have this same discussion again.