I don't think "they" (i.e. computer programs with intelligence) will ever exist, but assuming they did they could easily break out of our control, which was your first point.
Discussion
Interesting how the way we anticipate things unfolding is inverted, yet we arrive at the same place. Optimistic things will be ok??
From a historical perspective, super intelligence seems inevitable. There appears no reason to believe the trajectory of compute can't or won't continue. Because of this, I do genuinely want to understand if there is a real existential threat. But with nothing to rely on other than speculation, I tend to believe systems will self-correct to stay in balance. Nature FTW.