The reason why the belief in afterlife is not just neutral but harmful for modern people is that it demotivates them from doing immortality research.
While mainstream belief in an afterlife is probably a contributing factor in why we aren't doing enough longevity/immortality research, I doubt it's a primary cause.
Firstly, because very few people alieve in an afterlife, i.e. actually anticipate waking up in an afterlife when they die. (Nor, for that matter, do most people who believe in a Heaven/Hell sort of afterlife, actually behave in a way consistent with their belief that they may be eternally rewarded or punished for their behavior.)
Secondly, because the people who are in a position to do such research are less likely than the general population to believe in an afterlife.
And finally, because even without belief in an afterlife, people would still probably have a strong sense of learned helplessness around fighting death, so instead of a "Dying is sure scary, we won't truly die, so problem solved, let's do something else." attitude, we'd have a "Dying is sure scary, but we can't really do anything about it, let's do something else." attitude (I have a hunch the former is really the latter dressed up a bit.).
Secondly, because the people who are in a position to do such research are less likely than the general population to believe in an afterlife.
On this particular point, I would say that people who are in a position to allocate funds for research programs are probably about as likely as the general population to believe in the belief in afterlife.
Generally, I agree - it's definitely not the only problem. The USSR, where people were at least supposed to not believe in afterlife, didn't have longevity research as its top priority. But it's definitely one of the cognitive stop signs, that prevents people from thinking about death hard enough.
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