Can't really add anything so belatedly, franky1's response, while not exactly the scenario I had in mind, describes just how complex it would be to even mount such an attack. The preparation, the sheer breadth of collusion -- word would get out before they could put the resources in place, and the moral Ethereum devs and nodes would ensure a quick abortion.

But yes -- even if they succeeded, as many people have already considered (I remember Antonoupoulos describing the aftermath of a potential attack very well many years ago) -- a reorganisation would undo the attack. Remember, success isn't one single 51% attack to create an altcoin. Success means also convincing everyone else the altcoin is the one everyone would follow. Colussion also involves conversion.

Economically non-feasible is vastly underestimated.