Just imagine that tomorrow all the big mining farms disappeared. What effect would that have on btc? After the difficulty had settled, absolutely none. Correct me if im wrong but if there was a handful of compuers running the mining software then life would all carry on as normal.
I understand your thought, but you should know that mining difficulty requires a certain number of blocks to be produced (2016 blocks). If lets say 90% of miners decide to quit for some uknown reason, the difficulty will not change instantly. If the miners in this case leave early, right after the difficulty adjusts, then blocks will take hours or even days. I don't know the calculations exactly, but probably a hard fork will be required to change difficulty to very lower levels that will be able to sustain the network.