I don't follow. Even if the current group of miners decided older blocks are irrevocable, if there was a prolonged 51% attack, it would mean the majority of min

I don't follow. Even if the current group of miners decided older blocks are irrevocable, if there was a prolonged 51% attack, it would mean the majority of miners would be saying, "Hey, those confirmed blocks... that's not what actually happened. Look, here's the real history, with the real confirmed blocks." And then everyone else on the network would say, "Crap, who should we believe? Let's go with the majority." And the attack would win.