Something else came to my mind:
If both chains continue to exist, would it be possible to do 'merged mining' on both blockchains,
similar to merged mining of BTC and NMC today ?
No one has answered this. The answer must be yes, it would be possible with a modified client.
Another scenario comes up when you consider the shadow miners, all the at-home ASIC owners who turned off their machines as they lost out to the mega-miners. They'd surely consider turning them back onto mining on the "losing" fork at reduced value, because the difficulty will drop dramatically as well. Shadow miners are mostly from the idealistic phase of Bitcoin, so their incentive to continue on Core if XT wins out will be great.
Both forks continue, one will not die to zero. One may become sub $1 again, but that's OK for some people.