It would have set back ASIC mining on Ethereum by about a year had it been deployed immediately. From what I understand, the current proposal is to switch to ProgPoW in June, which gives hardware manufacturers a full 6 months to come to market, and the ProgPoW algorithm has already been studied by numerous development teams as it’s been around for many months - hardware manufacturers would not be starting from square zero.
The key technology for ProgPoW is likely to be either HBM2 or HBM3, something that we know existing cryptocurrency ASIC developers were already exploring for Ethash, which means some of the bigger barriers have potentially already been overcome.
That is to say, because the switch is happening so late, I no longer believe you’d get a years worth of advantage out of it. I don’t know that you’d have ProgPoW ASICs as soon as June, but you’d probably have them before 2019 ended.
Historically I think communities have been pretty bad at recognizing when they have secret ASICs mining on their networks, if not in part because they simply don’t want to believe it. I have 3 examples that I am quite confident about: Bitcoin (the S9 was mined secretly for about 4 months prior to launch, I believe), Monero (some sources have suggested to me that the secret mining went on for >9 months before detection), and Sia (there’s blockchain evidence that Bitmain secretly mined Sia for 2 months prior to announcing their ASICs). I also believe that Zcash had secret miners on it, however this is contested and I don’t think there’s enough evidence to definitively say one way or the other.
Even if you do find evidence, then you have to pull the political strings required to execute the fork, and as Ethereum matures that’s likely going to become increasingly difficult. My guess is that secret ASICs would be free to mine for 3-9 months even after being discovered, simply because it would take that long to motivate the Ethereum community around another fork.