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.
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.