The thing is that someone saying "I don't like the new UI" doesn't say anything. Always when you change something that more than 10 people use at least one person will be unhappy with a change.

What they hopefully do is check the metrics, e.g. of how many people switch to desktop mail clients and basic html view.

And honestly the new UI doesn't disrupt my workflow that much. You can like or not like the roundedness or that it got slower, but the workflow can stay almost the same. Therefore I'd say the current change is at least better then when they switched to teh Google-plus-everything mode a few years ago and fucked up all processes and all static links.