Well they have two geniuses, a cosmic level bloke, one of if not the best healing factor in the MU (not to mention Sinister's intellect). They also have two of the most powerful mutants in the world, and both those guys are fairly intelligent as well. They win
They could probably just plug Xavier into Cerebro and take down a few million minds, before they even start a proper attack
I agree, but I think you are underestimating Xavier and Magneto here. These guys are in the upper class of powerful mutants Xavier could mind rape pretty much most of the planet, and Magneto can hold his own against the survivors, it's the cosmics they would struggle with
Unless Xavier has Cerebra (or Cerebro, what the hell are they calling it nowadays?!?!) then he isnt powerful enougth to take out the entire planet Not even close.
Magneto stands no chance even with prep against cosmics
Neither of these guys are strong enougth to take on cosmics
with prep, most of those heavy hitters of the mu, like rkthor and ss would be neutralized. thanos richards and doom, could do this alone just as easily as this whole group. and them alone are more liable to not screw it up... i think magneto and xavier can be useful, but are not necessary, and i doubt there is much sinister can do that thanos cant do, or figure out how to do...