There's just not a whole lot of damage you can actually do with 51% of the hashpower. Block other people's transactions? The transactions will get through the instant you lose your 51%.
Why would the attacker stop the 51% attack? Say you are Paypal or VISA or a government agency. Destroying the network is like squashing a bug. You built up 51% to destroy the network. It is a pretty fucking trivial investment from your perspective. Maintenance cost of the attack is trivial. There is just the upfront capital cost. In addition, by selling on the way up or postponing the attack date, the investment partially pays for itself. Why would the attacker allow someone else to recapture 51%? Why would anyone invest the resources necessary to recapture the network from a determined attacker? It is not likely to work since the attacker can invest resources too. Even if the martyr is successful, then he will still have wasted a large amount of money. I am skeptical that anyone will fuck themselves royally to save the currency.
The network's days are numbered.