Then they can just implement Segwit. I'm not really seeing the problem here. One version of Bitcoin works today, the other doesn't. It's not really that complicated.
It was hard enough to get the support required to implement SegWit on Bitcoin proper, even though it had a far more immediate need for it. Bitcoin Cash was literally founded for the purpose of avoiding SegWit and making the Lightning Network impossible. The idea that all its supporters will suddenly see the light of SegWit is implausible.
Most Bitcoin Cash supporters supported Segwit, they just didn’t support Core artificially limiting the block size to force people to use their proprietary solution and then lying about it.
Then they can just implement Segwit. I'm not really seeing the problem here. One version of Bitcoin works today, the other doesn't. It's not really that complicated.