Two primary concerns, technical feasibility and political strife:
I have my extreme doubts that you can move a chain like this without causing it to collapse. As yet all we have seen out of the eth camp is more broken proof of concepts -- not a viable model for a potentially trillion dollar economy. How to you replace a jet engine mid flight? (You don't. Unless you like not safely landing).
PoS coin is worthless coin. If you want to have your expensive-to-mine gas coin be worth something, you have to make it hard to acquire. My second concern is that if they do manage to 'migrate', enough folks will ignore this and keep mining. This is a problem today with more contentious PoW hard forks.
Ultimately this behavior will lead to more chain forks, which unlike in the ETC days actually is a big deal today. Whos USDT USDC etc is the real coin? The eth1 PoW 'legacy' network, or the eth2 'pos' chain, or what about the eth1-a/eth1-b fork when the first political staking challenges come up (see all world religion schisms). PoW solves this problem. One truth, enforced by universal energy usage. Not power players arguing over interpretations of religious text.