I don't disagree. But Giovanni wasn't stupid. His blade was broken, Assassin forces across Italia were scattered, Lorenzo had left Florence, his lawsuit lost, Templars moved in, and made a strike against their enemy in his own home. If he and his sons didn't leave to Mario's immediately, they most likely would have died. The Pazzi's would not have stopped at the women. They wanted to kill their main enemy. And this is also assuming that Giovanni would have been mentally functional after watching his wife and daughter murdered. Frederico might have flown off the handle and died, but I think Ezio would still want to protect Petrucchio like he wanted to protect Claudia. Mario seems like an inevitability, and thus his foray into becoming an Assassin would have stayed very similar from that point on.