He is not saying Europeans are 'dominant for ever'. He is saying that because of geography, etc... that Europeans developed technology faster, which gave them a head start, and that head start has perpetuated, exactly because that is what a head start does. Not that it can't even out in time, like a ripple.
I think his thesis is totally compatible with saying "if New Guinea" had developed technology sooner, they would have dominated and killed everyone. It isn't like there isn't evidence of wars and slaughters in Polynesia, they just didn't get the technology.
What I took away from the book is not that "Europeans" are evil (which would mean they are genetically pre-disposed doesn't it?), it is that all humans equally dominant whenever and however they can, and technology heavily tips the scales. And Europeans just lucked into it.