They've actually forked Ethereum to keep the original mined version, ETHPoW ($ETHW). They'll keep mining on the original PoW chain as long as it remains profitable.

https://blockworks.co/proof-of-work-ethereum-fork-pow-rally-...

I'd expect the value of ETHW to crash fairly rapidly, though, because there are not many buyers interested in buying into a deprecated chain without official support or ecosystem buy-in, and lots of sellers who need to sell their mined ETHW to fund mining operations. Then we'll see miners shut off and leave the network, as the mining rewards can no longer support the electricity costs of mining. At some point it might get 51%'d, but at that point nobody will care.