I dont think people quite understand the amount of gpus are mining eth. If, and that is a big if this comes out you cant take that amount of gpus and just switch mining to another coin. Yes in theory you can but that will crush diff in a massive amount of those coins.
This works both ways. Massive number of GPUs mining Eth means that adding a few thousands ASICs wont affect the difficulty at all. Even adding a hundred thousands ASICs wont render GPU mining non-profitable. I wonder how many ASICS Bitmain can produce. There's a shortage of DDR3/4 chips so they will be limited by the number of chips they can source.