>The problem is not that longtermists go around saying "We really ought to do more genociding," it's that longtermists go around arguing that essentially any present-day sacrifice short of human extinction is trivially easy to justify according to their deeply wonky assumptions about the future.

I don't think they do this either.

Like, if I go to the EA forum (probably the online community with the biggest longtermist population) and look for highly-upvoted posts about longtermism and ethics, I find things like this and this and this and this. There is a lot of skepticism about naive utilitarianism, and an abundance of people saying things like "being a hardcore utilitarian can lead you to weird places, therefore we should find a theory of ethics that avoids doing that while keeping the good parts of longtermism and utilitarianism intact". Conversely, there's a total lack of posts or responses that take the opposite stance and say, actually, we should accept literally every crazy consequence of naive longtermism, and it's completely morally okay to sacrifice millions of people if it reduces the odds of humanity's extinction by 0.01%. Seriously, I swear I'm not cherrypicking examples, this is what the average post about longtermist ethics looks like.

You insist that longtermism is intrinsically built around exotic hypotheticals and a willingness to make horrible sacrifices--but if that's true, then why do they spend even more time picking those claims apart than their biggest critics do?

I think you could reasonably argue that longtermists need to spend more time working on the philosophical foundations of their movement, to find a way to reconcile the good parts of utilitarianism with the bad parts (and I bet most longtermists would agree!). I think you can't argue that the core of longtermism--"the view that positively influencing the long-term future is a key moral priority of our time"--is a stance that can only be justified by the bad parts.