They checked that and power users didn't customize that much. There's a lot of studies that show that power users don't customize, since customizations are not portable across PCs/systems in general.
I wasn't kidding when I said they went through a lot of numbers while designing the ribbon (I read the whole blog archive at one point).
On top of that, the new UI is way more discoverable for new and intermediate users. And at the end of the day, beginners are let's say 10% of the users, intermediate users at various levels are probably something like 85% and power users are probably only 5% ;)
