If it is possible for any single player to manage to amass that much computing power relative to the rest of the network, it seems to me that the whole proof-of-work concept is invalidated, fundamentally. We're just back to human webs of trust relations. Those who then claim that bitcoin has been hacked would be right to do so...and perhaps it would be best to abandon the block chain concept altogether.
Having said that, I believe it is possible to modify the proof-of-work algorithm to make it less likely to favor people with a particular type of specialized hardware.
Umm yes, proof-of-work might lose all credibility, but there would still be proof-of-stake standing untarnished. No need to get rid of blockchains altogether, that is throwing the baby out with the bath water.