I like how people automatically assume change == good. Maybe I'm getting old, but it seems to be a young person thing (as is the rewrite everything from scratch mentality).
Change is change. It can be good, it can be bad. I'm not an expert on such things, but from everything I've read, the change to systemd is bad. And it seems to be a bad change in much the same ways the examples of change you gave (Metro, Unity, etc.) have turned out to be bad.
The Unix philosophy has always been to do big things by using little pieces. To violate this philosophy is not necessarily bad, but it would seem like trying to fit a round peg into a square hole. Sure, if you hammer it in hard enough, the thing will fit. But your square hole might have trouble fitting square pegs through afterwards, and your wooden board might crack after you fit more things through the hole irrespective of shape.
I'd have used a car analogy, but the best I could come up with is using the wrong kind of motor oil, which when put that way, doesn't seem quite as severe as the systemd problem.