[dave7]

For Bitcoin specifically (by far the largest), the amount of computational power is now very large indeed. An argument often seen against Bitcoin is that of it's environmental damages - quotes like "it uses more power than the whole of Denmark!".

So yes, 51% control could be usurped by a dedicated attacker - but the resources are no longer trivial. And as noted, this "control" doesn't buy you a great deal unless you keep it going for eternity, and for the time period you do have control all you can do is basically stop it working properly by preventing new transactions - rewriting history is exponentially more expensive the farther back you try to go.