… Isn't this basically what Stephen Hawking said a year or two ago, only more verbose?
Also, people automatically assume that "Europeans had guns, therefore automatic win!" when that typically only applied in cases of large Native American charges at fixed positions… Normally, the tribes didn't fight that way, a lot of hit had to do with the plague that swept through the Americas in the… Mid 1500's I think? Wiped out something like 93% of EVERYONE living in North America. Kind of made it a bit easier for the colonists to sweep in. This may or may not have been caused by early European contact, some evidence points against it, some for it.
It's telling that the Vikings had the colony in Greenland running for about 500 years, made numerous expeditions to America, and didn't so much as make a FOOTHOLD. The vikings, go-to culture for Crazy Badass Invaders, were getting kicked out.
And I personally think that would apply to most of the ideas of an "alien invasion". The only actual threat to humanity as a whole would be a full-scale, wiped-out-everyone-from-orbit campaign. As a species we're a tough, clingy, and downright verminous in the right conditions. Any type of alien attack wouldn't come in, blow up some monuments, and have the governments of the world in unison say "We give up!" If there was any sort of ground action, we would fight back in a guerrilla campaign that would make any war in human history look like afternoon tea. Their armor is too tough? We get bigger guns. They have shields? We figure out how to neutralize/overwhelm/bypass them. They have lasers? We put mirrors on our shit.
Disease wouldn't be a factor one way or the other. Whether from biological difference, hazmat suits, whatever, the biggest threat we'd face would be an initial shock-and-awe strike. Granted, they could wipe out a very large amount, especially if the focused on civilian population centers rather than a military take-over, but if they were going for anything short of near-simultaneous, worldwide genocide… Even if they "conquered" Earth, their grandchildren's grandchildren's grandchildren would be hearing about the latest batch of "last remnants of humanity" that were still around messing with shit.