It's the same as with any major UI alteration.
- People familiar with the old UI have to re-learn and that's a considerable downside for them.
- New (and less experienced users) have fewer adjustment to make.
Familiarity with an old interface (or just another interface) may already bias you against change.
Objective usability testing is really hard. Hey, even just working out what you need to test is hard. What is more important? That users can do repetitive tasks quickly or that they can find powerful features quickly and/or intuitively? And what about making features easy to train?
A lot of people (perhaps the OP included) seem to assume that Microsoft's UX changes are there for the sake of change. They may well be... But the developers do a pretty decent job of explaining the need for change. And they claim it's a tested benefit.
If you don't like it, consider that you're not every user. Perhaps there are simply just more people who need a different tool to the one you need. If Microsoft can detect that, they're going to gear things to the bigger market, every time.