I'm the first one to defend a simplified design for entry-level to
average users even if it doesn't support expert features. But this
kind of design should be only for new features and never, ever be put
in place of a previous design already in use. This is the golden rule
of computing - if ain't broke, don't fix it.
But our current UI is broken in many respects! Thing is, motivated users can wrap their heads around almost any UI, as shown by the fact that vi and emacs still have such a strong following, to mention just one example.
The fact that human brains posses such a high plasticity, however, is no excuse for us not getting our act together. Whenever you change a UI, you will break someone's habituation to that UI. Unfortunately, habituation is a very complex issue: For example, people can be very creative while working around a program's limitations, and this often involves using the program in ways that were never taken into account by its original designers. This means that, given a large enough user base, even changes that appear completely innocuous will likely end up irritating at least some users.
Braking people's habituation will always cause problem, but absolutely necessary if you want to make progress.