Syracuse and Pittsburgh had announced they were out of the Big East a whole-ass year prior to ND's ACC move. That league was dead. ND would've happily parked their non-football sports there forever had the league remained viable, but it was not. (Also, football is all that matters, as we all know, so ND leaving the Big East in not-football had almost no effect on that league.)
As for your second paragraph, this canard has been tossed around for several realignment cycles now, and it's just wrong. First, the idea that it's somehow Notre Dame's responsibility to stop the B1G from destroying other conferences or other schools from repeatedly shivving each other is so incredibly funny and rich I don't even know how to respond. Second, just for one of many possible examples, literally nothing about the demise of the Pac-12 would've been affected or stopped in any way by ND being in the ACC for football.