That seems like too much of a simplification. Neither India nor China were especially vulnerable to European diseases yet they still came to be dominated.
Yeah, but that was centuries later, and probably in part due to the riches gained in the New World.
China especially was only dominated well into the late 1800s. You can see that more as a symptom of Chinese decline. Note that Japan made the transformation to modernity quite well.
But you're of course right in that we'll never know what would have happened. The Americas were way behind in military technology, but they could have caught up in a generation or two, with huge amounts of gold and other riches to trade.
