ETH is no longer GPU only.
As a different point. A lot of people believe in crypto and will still build rigs when profits are down but I dont think thats whats going on in your example.
I don't think there is any ethash ASIC miner on market yet? Bitmain is still developing it.
Publicly Bitmain's ETH ASIC miners are still in development phase, but I guess that some people are not believing that, at least not in that context. Some believe that Bitmain has already made ETH ASIC miers, and that they has been mining ETH with their ASIC miners secretly. If you want to read more about this, go check this discussion
https://bitcointalk.org/index.php?topic=3162392.msg32929859#msg32929859But no matter what is true, its just a matter of time before we would be able to buy ASIC miners for ETH or maybe some other crypto as well. When those miners come out, I guess GPU mining for that coin will die.