Why would they take action to exterminate anyone, when they can simply ignore them and let them live and die in extreme poverty, unable to participate in the economy? The west already accepts this kind of poverty with the sweatshop workers, and nobody bats an eye.
It's just disenfranchisement. Only the people who try to use violence to replace the political leverage they lose when they lose economic leverage would need any action taken against them, and that's easy to justify as self defense.
Because 7.5 billion people with nothing to lose would quickly turn chaotic and mass-murdery. 90% of humanity in idle poverty is impossible to justify, at least today they do something of value that can be used as a retort by the morally unscrupulous. A black-and-white situation where a few get everything, most get nothing and do nothing for anybody else is beyond justifying, so the self-defense argument wouldn't work.
It's already been here for much of the 20th century, and everyone seems to have just accepted it. We're just going to have Europe and America join sub-saharan Africa.
I think it's inevitable.
The moral justification for the existence of poverty is one of purpose and worth through contribution to society. The family member of an ultrarich baron can sleep at night mostly due to the idea that those who are rich contribute value to humanity as a whole, ergo those who are poor simply aren't working hard enough or contributing in a meaningful way ("just world fallacy" etc)
When AGI becomes reality, that reasoning disappears. When everyone's worthless, no one is. Thus the scenario you speculate about would need everyone in the upper echelons of society to be consciously, openly sadistic power-hungry people, which is not the case even when taking into account the higher dark triad rates among them.