if you could in fact manage to steal or borrow 1 million bitcoins and then double spend them on an exchange and transfer them for another coin you wont get any argument from me.. if someone managed to do that it would be catastrophic.
however you're talking 600 million dollars here.. I'm sure someone would notice your double spend attempt.

the point is though that in order to do that to bitcoin you would have to hack 2 places and pull it off before the network could react.
and I would be very very surprised if the network failed to counter your move.
if they did however fail then sure the only other option would be to try and hard fork you out...  which would be a very difficult and costly process in itself given the size of the network.

I agree that would be theoretically possible to do on bitcoin but  given the sheer size of the network and the fact that you have to carry out two successful attacks... it would far less likely than attacking a small PoS coin where you need only hack one place and then you can pretty much use those coins to wreak havoc on the network as many times as you like once you get them.

if such an attack happened on BTC it would probably be cheaper to just hire a small army of mercenaries to hunt you down and make sure everyone who holds the private keys to those stolen coins is eliminated.

but now we are getting well into the realms of pure fantasy lolz.