These mining restrictions are... interesting, given that Ethereum is about to move towards staking rather soon anyway ..
I think it's largely political, getting stake pool nodes and people using ETH2. There will likely be ETH1 and ETH2 running side by side for some time. ETH1 can be burned to get ETH2, so there's a one-way transition (though I'm not sure if this story is complete yet, that's the plan anyway).
The earliest likely merge date is Q1 2022. The only exception is if the situation is urgent (miners attacking, or nicehash having a dangerously high percentage of hash) - in this situation the merge itself could be done in a month I think.