Earlier quoted context omitted.
Repurpose it how? You'd need ASICs to mount a 51% attack. Using a few quickly googled numbers, you'd need ~56EH/s to double the network and mount a 51% attack. An S9 does 14TH/s, so you'd need ~4 million, at $3k each. So around $12 billion (plus the datacenter, operations, etc). You'd pretty much send BTC to being worth $0...and then you're stuck with $12 billion in useless hardware.
If my goal is to re-purpose, I would use a General Purpose Computer The Market Cap of Bitcoin is $184,000,000,000 I have plenty of money to play with.
Moreover, to gain $184B by shorting Bitcoin you'd need to find someone willing to loan you $184B worth of Bitcoin, and then somehow sell all of it without crashing the price or drawing unwanted attention to your activities. Of course if the price doesn't react the way you expect you could end up paying a lot more than $184B to buy it all back to repay the loan. You shouldn't assume that even a successful (but expensive, and obviously temporary) 51% attack will drive the price down to zero.