I think the problem is primarily that a superintelligence would be fundamentally inscrutable to us, i.e. we don't know how it would think or what its goals would be. It might decide that humans are a minor inconvenience to achieving its goals and thus worth removing. Or that burning all carbon lifeforms could power its GPUs for a week.
Even if it wouldn't want to do this at first, the fact that it'd have the capability to seems bad.
Even if it wouldn't want to do this at first, the fact that it'd have the capability to seems bad.