What makes you think merchants won't switch to xt if the majority of miners mine on xt and not on core?
Would it not be in the merchant's best interest to accept coins on the chain that is more time efficient?
Coins being sent on the old chain would be super slow. Can't see why merchants would not switch of the majority of miners processing transactions moved to xt.
A chain with fewer miners is not more efficient. It is more secure, though there would be very few things that could attack even 25% of the network's hashing power.
Blocks would be slower (25 minutes on average instead of 10), but this would only last for a few weeks until the difficulty goes down. Even today it's common for blocks to take 25-60 minutes, so this wouldn't be Earth-shattering.
This wouldn't actually last for long in the scenario where 75% of miners switch but no big economic actors do because the miners who switched will switch back very soon after they realize that they're mining coins that no one will accept.
This is of course assuming that ZERO merchants choose to accept XT fork coins.
That has yet to be seen. No one knows that.
Should be interesting to see what exchanges/businesses do when some accept XT and some dont (assuming that scenario plays out).