What could they do if they are really malicious?

Sell it for zero? Or just long-term downward pressure on price?
-> I'd love that

Sell it for (trillions) to the central bank?
-> I'm ok with this; it recognizes its value

Buy an army/resources and fight the virtuous Bitcoin users in the meatspace or via regulation?
-> I'd argue that the current States & Central Bankers govern much more coercive powers in their hands. So what...

The size of your stack does not matter.