The reason I'm being a pain about this sort of stuff is that I have an allergy to signs that our tribal beliefs - atheism, physicalism, MWI, memetics, reductionism etc - are being used as synonyms for rationality.

Agreed. To pick a more extreme example: Tribal beliefs here seem to also include cryonics, life-extensionism, and immortalism. While I agree with these as desirable goals, if feasible, the frequent assumption here on their practical feasibility seems to rest on assumptions of continued economic, technical, and medical progress, which are actually contrary to recent evidence. (To put my own cards on the table - I'm an Alcor member, but more out of status quo bias from the 1990s, when it looked like Drexler/Merkle nanotech was going to be funded and succeed and be applied to medicine on a timescale of a decade or so. Didn't happen.)

Quoth Peter Thiel: "The single most important economic development in recent times has been the broad stagnation of real wages and incomes since 1973, the year when oil prices quadrupled. To a first approximation, the progress in computers and the failure in energy appear to have roughly canceled each other out. Like Alice in the Red Queen’s race, we (and our computers) have been forced to run faster and faster to stay in the same place."

An alternative to the consensus view on LW, equally physicalist, is to see significant further healthy life span gains as unlikely. Under the practical technological and medical constraints that we face, it might be more helpful to look towards easier assisted suicide than towards cryonics and similar options that rely heavily on progress that has in many ways stalled.