They would leave a gap on the real Bitcoin hashrate, which would mean that someone mining Bitcoin Cash would go back to mining real Bitcoin, because the difficulty would lower and the price would still be several times higher than Bitcoin Cash. That is the irony, at the end of the day Bitcoin Cash can never get anywhere near the real Bitcoin. As soon as people leave the real Bitcoin to speculate with the Bitcoin Cash hashrate, they will create a new opportunity to mine Bitcoin. It's a constantly leveling-itself system.
Yeah - but it will take at least two weeks for the bitcoin hashrate to adjust downwards, and in the meanwhile a bunch of noobs who don't understand what is going on will be panicking about their transactions taking forever to be picked up, and we'd get some panic selling on the exchanges. If the panic selling results in the bitcoin price dropping some more, the profitability of mining BTC drops further and more miners switch. Rinse and repeat.
If only we'd just got large blocks a few years ago, we'd be in a different place. Oh well - have kept most of my BCH, so it makes no difference to me which coin takes the top spot.