It was known that it would have a different processor architecture, however historically backwards compatibility was a given if you had any entrenchment worth preserving. Usually it is achieved in whatever way is necessary: a secondary modern-process, lower-power PowerPC coprocessor chip for older games, or a real-time binary rewriting system, etc.
I am surprised that Microsoft would throw the lead their 360 has amassed.
I don't want to juggle multiple systems in the entertainment setup. If I switch to either a PS4 or a Xbox One, either leaves me out in the cold with my old "investment". So any advantage the One had in that regard is gone. It isn't a huge deal at all, but is factored in or not.