[Responding to Alex HT above:]
I’ll try to find the time to respond to some of these comments. I would strongly disagree with most of them. For example, one that just happened to catch my eye was: “Longtermism does not say our current world is replete with suffering and death.”
So, the target of the critique is Bostromism, i.e., the systematic web of normative claims found in Bostrom’s work. (Just to clear one thing up, “longtermism” as espoused by “leading” longtermists today has been hugely influenced by Bostromism—this is a fact, I believe, about intellectual genealogy, which I’ll try to touch upon later.)
There are two main ingredients of Bostromism, I argue: total utilitarianism and transhumanism. The latter absolutely does indeed see our world the way many religious traditions have: wretched, full of suffering, something to ultimately be transcended (if not via the rapture or Parousia then via cyborgization and mind-uploading). This idea, this theme, is so prominent in transhumanist writings that I don’t know how anyone could deny it.
Hence, if transhumanism is an integral component of Bostromism (and it is), and if Bostromism is a version of longtermism (which it is, on pretty much any definition), then the millennialist view that our world is in some sort of “fallen state” is an integral component of Bostromism, since this millennialist view is central to the normative aspects of transhumanism.
Just read “Letter from Utopia.” It’s saturated in a profound longing to escape our present condition and enter some magically paradisiacal future world via the almost supernatural means of radical human enhancement. (Alternatively, you could write a religious scholar about transhumanism. Some have, in fact, written about the ideology. I doubt you’d find anyone who’d reject the claim that transhumanism is imbued with millennialist tendencies!)