And who would engage in this 51% attack? The miners?
The miners are one of the biggest groups that SUPPORT Bitcoin Cash, so I find it unlikely that they would spend millions attacking it.
Anyway. It matters not. The people that want Bitcoin to be a settlement layer are free to spend 50$ per transaction to use Bitcoin, and those of us who want to use it as an actual peer to peer electronic currency will use Bitcoin Cash.
Everyone wins. It doesn't really matter what is the "real" Bitcoin.