I don't think Ethereum will ever switch. The miners control the chain now, POS will disrupt that. The miners can't make an orderly exit of resources or to a new coin so they will persist on the dominant PoW-Eth while PoS-Eth loses all chain resources.
If the POW miners continue mining, that’ll create a fork which will quickly become unprofitable because of the difficulty bomb, and all of the stable coins on the fork will become valueless rendering the fork useless.
"If all miners do not make the switch to Proof of Stake, then there is the danger that ethereum’s blockchain might fork. A similar situation occurred in 2017, when bitcoin miners forced a fork in its blockchain by throwing their weight behind bitcoin cash. Ethereum’s founders, however, foresaw such an eventuality and programmed its blockchain to incorporate increases in difficulty levels for its mining algorithm"
