This is never gonna happen, but interesting to think about: if I understand correctly, the bitcoin network difficulty adjusts every 2000 or so blocks, in order to keep blocks being generated once every 10 minutes.
Now at current network hash rate of 130 TH/s (well, more like 150 TH/s recently), the difficulty is 15,605,633. What if suddenly the network hash rate drops by 90%? Although the difficulty will adjust downwards to compensate, that difficulty will take 2000 blocks to "readjust" since it doesn't happen instantaneously.
But if the network hash rate is reduced by 90%, it'll take a looooooong time to generate 2000 blocks before the network difficulty compensates. Is it feasible that a sudden, extended drop in hash rate will lead to a long, dry spell due to the lag time before the difficulty drops in response?
One more thing to note, difficulty cannot go up or down by more than a factor of 4x per retarget, regardless of hash rate changes. So if the hash rate is reduced by 90% right after a retarget (worst case scenario) and then remained constant, one retarget would not be enough to bring the difficulty in line, it would take 2 retargets and unless my math is wrong close to 6 months total time.