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.
1. You completely ignore sociological & geopolitical aspects of such an event. Such a large operation would be a direct proof that Bitcoin is a danger for current establishment, which could cause MASSIVE surge in publicity for Bitcoin, which would bring a lot of new people into mining business.
Every action has a reaction, you simply CANNOT ignore that.
Let's consider a scenario. Let's assume that FBI/CIA/Other US govt agencies start a new supercomputer, and gain 60% in one day.
But there is no action without reaction. China & other countries who don't like USA immediately assume, that Bitcoin is probably a danger to US dollar and because China really wants to get rid of dollar, they put one of their newly built supercomputers into mining Bitcoins, so dollar will fall even more quickly.
Also, other "wild" miners join the game - some (ideological) to protect Bitcoin, but most because
USA admitting that Bitcoin is dangerous confirms that is it valuable.
So not only the attacker's network share drops below 40 or even 30%, but Bitcoin value skyrockets.
2. Increasing inflation by sunsetting old coins is in no way solution to this.
So your entire reply is decidely not on topic (as is mine, unfortunately).