Limited slot availability has been shown to be a false argument. A380 is so big and heavy that it REDUCES the number of available slots at an airport because of the spacing it requires on landing and takeoff. You can land two A350s and 777s in the time space required of an A380. Further, the double decks make A380 turn around times terrible. You can unload and then reload two A350s and 777s in the time it takes to turn around an A380. And the A380 also increases congestion INside most airports.
I would respectfully disagree with your arguments here - do you have supporting facts?
I am not sure of takeoff separation, but think gate times are not as bad as you say. Have you ever been PAX onboard? Loading at SIN was impressive and the terminal was well run, same experience in SYD. I think LHR needs the A380 to be able to maximise landing slots in coming years as they are pretty maxed out.
A380 works very well for the major long haul players, and as a post above 90% capacity on Emirates must be a money spinner for the bean counters!