The points in favor of the 380 are the reduced noise level and the reduced cabin altitude. Both of which do matter on long-haul flights, whether you want to believe it or not. You simply arrive in better shape.
Don't forget the extra space per passenger too. A roomy ride is far more comfortable than being squashed up against one's fellow travelers, and makes everyone simply feel better about the experience.
Quietness, roominess and low cabin altitude were supposed to be attributes of 787 too, but most airlines squeezed in an extra seat and omitted the heavy sound insulation in economy. Result - no net passenger benefit, other than a better cabin pressure.
I think Airbus with A380 and A350 have got the size pretty right - the airlines have been reluctant to squeeze in extra seating, presumably because it really would be taking the piss to do so. With Boeing's products they seem to be able to just about get away with it, so they do. For example, a 9-across 777 is far nicer than the usual 10-across that most airlines fly.
The best sort of business for an airline is regular passengers who are loyal. Providing a superior ride is absolutely the most reliable way of achieving that, especially if the price differential is negligible. Having built up such a business, the airline would be taking a big risk if they renewed their fleet with aircraft that revert to cramped and noisy, for then the only difference left to discern between airlines is price.
In the case of Emirates, they've built a huge business with A380 being a big (and popular) part of that. Swapping A380 for, well, anything else is potentially going to be a capacity reduction and (depending on what was bought) a ride quality reduction too. Both would be unappealing propositions for Emirates and loyal, regular passengers alike.
If the capacity reduction was unavoidable (i.e. no extra airport slots), a 777X would have to be about 33% more efficient than an A380 to generate the same profit per flight as an A380. Sounds unlikely, especially if RR keep improving the Trent 900.
If capacity can be preserved (i.e. slots are available, so two flights can be flown instead of a single A380) then 777X sounds like the wrong aircraft - it'd be too much capacity. Two A350s or 787s would be a better match than two 777Xs. Given that the A350 is reportedly very comfortable and 787s (when fitted with 9-across seating) aren't, buying A350 sounds like a safer bet for Emirates in the round. Unless they're able to persuade Airbus to stretch or pep up the A380.