Well, the current chain that everyone is following does still have the 'ice age' in it that come September will start making mining new blocks take exponentially longer to mine. If the miners want to continue with the POW chain, they'll have to coordinate a fork of their own to delay or remove the ice age. And that there is the rub. The ethereum miners have never been very good at coordinating amongst themselves.

And asset-backed stable coins like USDC will not be redeemable from the GPU chain.

The energy consumption debates are a distraction from innovation, so many want The Merge to happen soon to get past this energy issue.

Ethereum "classic" is still around for the "code is law" diehards. I'd expect most of the people who are fine with the current ETH hard-fork chain will probably be fine with the merge.

It'd be interesting if we did end up with three live chains though, probably not sustainable.

It's quite likely that the old Ethereum chain will almost completely die off because although noone can force anyone not to fork it, the transaction fees on the new network will be a fraction of the old one, so there will be a big incentive for people to use the new chain.