If, as ShardPhoenix says, "quantum multiverse is for decision-theoretical purposes equivalent to having an infinite ensemble of classical universes with small differences between them, plus self-location uncertainty", (BTW I would use "similar" instead of "equivalent"), "posts about infinities and death in many worlds, and about whether a copy of you preserves your identity" seem perfectly justified, so I find your comment puzzling as a response to his comment.

Is there reason not to take it seriously, from a quantum-mechanical perspective?

I may not agree with their conclusions, but I'm fairly sure the MWI worlds actually exist.