let's look into the history, we had bitcoin lower than $200, but to the best of my knowledge,
relating that to the electricity costs, mining will become a loss instead of profit
but to my knowledge we didn't see the 51% attack..
i am guessing the 'difficulty' implemented by mr nakamoto is a great filter for this kind of situation,
when there are no miner, the diff will go down, much much down even your android phone can do the mining..
and when the diff go down, a miner or two will start their miner to snap the profit as much as they could, automaticaly this will give the 'decentrilized network state',
thus preventing the 51% attack
but we never had an hard fork debate that will eventually turn real in few week, like this one, in case of an hard fork the bigger fork, have all the reason to kill the small one to destroy its competitor
if the diff go too much down, it will be even more easy to do a 51% because any entity that have a low hash could do it, do you think the random dude care about supporting the bitcoin network, the majority only want more fiat money....