Earlier quoted context omitted.
Just mine no transactions. After the network grinds to a halt the exchange rate will crater and then you can close out your short position.
It will almost certainly cost more money to obtain 51% hashrate than you could gain from a short position, by the time mining rewards aren't enough of an incentive. Even the NSA doesn't have enough computing power to achieve 51%.
10 years from now, what if we have a handful of bitcoin mining firms, similar to the current role played by market makers like Getco or Knight?
You can't just dismiss this point. You need to come up with some process that makes it more valuable for someone to be a small player in the network than to go all-in.