Besides loadings and profitability, I can only assume VS and Virgin Holidays have data that indicates that the vast majority of passengers on its LAS flights are only going to Vegas.
Did a holiday a few years back with VS where we flew LGW-LAS, had 3 days there (which is enough for most), picked-up a camper and spent the next 10 days doing canyons, deserts, valleys etc, ending-up in a campground south of LA, from where we flew home LAX-LHR.
At least if we did this in future it would all be out of 1 UK Airport!
Granted that no one really knows how long it'll be until previous levels of travel return, but VS used to fill sometimes 5 744's a day (LGW x2 daily at least, MAN up to 2, GLA/BFS) from the UK to MCO and I can't see that capacity ever coming back again - which means if demand does return (or even half of it) prices will be high! (but I guess that's what they want)