All that ETH hashing power will point elsewhere, maybe ETC. Look what happened to LTC in 2013 when ASICS hit the market for BTC. EVERYONE with GPU rigs started to mine LTC. All that competition drove the difficulty thru the roof immediately. This made it more costly to mine a LTC and LTC went from 2 to $55 overnight. the reason is Miners wont sell for less. they create the supply. Supply becomes dammed up from difficulty and miners unwilling to sell for less than a profit. Alls it will take is a two week lag from this phenomena and the #2 GPU coin will skyrocket from this effect.
Just my take, there are other factors but it is a sound observation/metric for long term profit.