The network would survive with a vastly reduced number of miners. But at that point it would become feasible for a state actor to wage a 50% attack against it and kill it for good? It didn't happen before because bitcoin was just a geeky curiosity; now that it's been proved to move real money, it has become a possibility.

Well in a bit of an ironic twist, because ASICs that are used for Bitcoin are kind of worthless without it, Bitcoin should continue to have enough value to run those miners at cost in some low electricity areas (assuming it doesn't entirely go under).

With a GPU mineable currency, if it crashes and becomes "unprofitable" to mine, people with GPUs can just use them for something else and the currency can die entirely.

So 50% attacks are still difficult because unless bitcoin becomes almost entirely worthless (at which point a 50% attack is kind of pointless), there is still just as much hashing power sitting around waiting to be turned on when it becomes profitable. Power that can't really be repurposed, so it's not really going anywhere.

It would be interesting to talk through that threat model, I think the over all network can be much more resilient to that sort of sustained attack than you're presuming.