They would. That is what eugenics is.
It's tangential to the main topic ("why people believe a biological super-intelligence is more probable than an artificial one"), but I think that what you said it's not warranted at all.
First, we know very little about the biology of intelligence: at present we are not able to explain the current variability in human intelligence, we have even less idea how to genetically enhance it.
Second, we share a psychological unity and genetically improved humans will presumably be grown within human family, so we have a much greater chance for them to share our values.
Third, even if no people get uplifted and the generational change brings about better people, it's still a net gain for humanity overall.
The existential risk of eugenics, super-people violently replacing normal ones, is the least probable, not the most probable, scenario.
I'm not saying we should concentrate on eugenics, I still feel that UFAI is a much bigger threat, I'm saying we should not avoid it because of x-risks.
I agree with all that, I was just running with the idea that one way or another, truly superior beings would in the end displace the rest, with or without actual war.
Now, successful breeding combined with life extension for all, so that present-day average people get to live into a future dominated by the results of several generations of breeding, that could be an interesting scenario for an SF story. I would expect the violence to originate with the marginalised rather than the elite.
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