Vitalik and Co are reacting by speeding up the planned fork to proof of stake which will cut out POW miners altogether.
If you are holding Ethereum on an exchange like Coinbase or some trust like Greyscale or 21shares, I wonder what happens in the case that it forks into multiple chains with significant market value.
Ethereum forked once already and the smaller chain (ETC) still holds about 0.5% of the value. In future forks it might be less unbalanced.
This applies to Bitcoin and all crypto currency in custody of course. It is just more imminent for Ethereum.
Vitalik et al are trying to force a fork, as they did when they left the original ETC chain.
Coinbase made the ETC forked coins available, not instantly but once they had time to do the coding to get it set up. Grayscale, who knows, you don't own any cryptocurrency with Grayscale. My guess is they'd go ahead and pull in those additional coins into the value of the fund, but you really have no guarantee of how they would do it unless they've put it in their terms somewhere.
All of the coins can be forked. Not your keys, not your coins as the mantra goes.