That wouldn't end Bitcoin. It might make some trouble, but even that wouldn't that big.
Even if you could drop the hashrate 50%(which is complete bullshit, since if it drops miners with older hardware just go in again), that just means double confirmation time. So what? It adjust after 4 weeks instead of 2 weeks, big deal ...
why would miners with old gear go in on high difficulty? The hashrate doesn't make a difference for them, the difficulty does.
Well, we assume a further decline as time passes eventually leading to everyone drop out.
Price fall, mining even more unprofitable for the reason of low price aswell as long blocksolving. It's a chainreaction that just needs to be properly started.