I am not assuming humans don't remain in control, but rather, I am not axiomatically assuming there's any inherent reason we will.
No, the argument is instead that the person claiming that humanity is going to be destroyed is making a fairly extraordinary claim and that requires fairly extraordinary evidence.
Or, in other words, we have tons of evidence already as for why the world ended is a fairly far out there prediction, given all the crazy people making these predictions keep turning out to be wrong.
So, you can make your extraordinary claim if you want, but really the burden is entirely on you to prove your extraordinary claim, and everyone else is free to remain on the default and completely normal end of the prediction spectrum, of believing that the world isn't going to end.
And when people do tricks like this, they are running away from the fact that they are making a wild completely out-there prediction, and hiding behind that by trying to come up with reasons as for why evidence doesn't matter and actually the burden of proof is shifted to those who have the default and boring prediction of the world not ending.
Its not a counterargument to basically anything, except as to avoid having to deal with actual evidence.
It is an argument that seems almost tailor made to have to ignore mountains of evidence against you.
In any other contexts the supposed "rationalists" would be fully in agreement that having evidence matters, and that not having any works against you.
So, in order to fight against this severe issue with their arguments, they have to invent a reason as for why the entire concept of evidence itself doesn't apply to them and they get to ignore normal evidentiary requirements.
We have precedents of people warning about existential risks in the past, when the warnings turned out to be false, or overblown. In some cases, such overblown warnings led to serious negatives for society (demonization of nuclear power).
Those counterfactuals are important, though. We do have a history of averted disasters, albeit not as large. Ozone hole, Y2K, think about things that seemed overblown at the time, and consider whether they were actually overblown or whether there was a concerted effort to successfully avert them.