Celine revitalized Vegas starting in 2004. That was the second wave. Profit was down thanks to all of the circuses (literally) and the Rat Pack leaving (that was the first wave) and she brought in Shania, Elton, etc. and de-stigmatized Vegas. Sis, what?
Vegas is built around celebrity residencies. It doesn't matter if they're the direct source of income (though entertainment is now 64% of Vegas's income) or just drawing crowds (see: their hotels doing better than literally anywhere else in America), etc. Without the residencies, they'd just be a flop like Atlantic City (who's also been looking to get in to big-name residencies since Britney signed on). Hell, they'd probably be doing worse than Atlantic City seeing as how their gaming revenues continue to fall at a rapid pace.