They can certainly do that, but that doesn't keep the merge from happening. It just means there might be a PoW chain on the side. It'll be up to the market to decide whether it's worth anything, and if it's not worth much, it won't support much mining activity.

So far, the miners don't appear to be organizing a chain split. And it would take some organizing; they'd need to defuse the difficulty bomb, get exchange support, try to get support from the people running tokens backed by custodied assets, etc. Perhaps most critically, they'd need to get a large portion of the user community to believe that PoW is better and worth sticking with. That's not happening either, and Ethereum users are very much looking forward to PoS.