More and more people own a quite powerful configuration. And so they wanted to have something good looking. MUI isn't slow on high CPU... at least i guess you shouldn't notice it. So gadtools and older GUI are not used anymore... or not too often. It also give a "general" looking for all your application.
Only "old" amiga freak with use 68000 or standard A1200 still doesn't like MUI for slowing down their system.
This new GUI could give a good alternative. I quickly test it with give example and it doesn't seems to be so fast... perhaps using less bitmap could help a bit