The value of the gear is just a question of the future profits it can generate. If that future revenue is less than what you get from the great short then it’s profitable. And that’s assuming you can’t reuse the same equipment for some other purpose.

It requires you to borrow bitcoins from someone -> sell them -> do the attack -> buy them back -> and then return them.

If you could borrow enough bitcoin, and gain enough hash, and crash the price enough for it to be profitable, it could possibly kill Bitcoin. Seems like an insane gamble to me though.

That said if I where writing a story I would have the protagonist gain the hashing power first, something strands their assets such as increased electricity prices making mining unprofitable or a next generation of hardware coming out etc. Then they come up with the idea of a big short.