I agree with your points about there being disagreement about EA, but I don’t think that they fully explain why people didn’t come up with it earlier.
I think that there are two things going on here—one is that the idea of thinking critically about how to improve other people’s lives without much consideration of who they are or where they live and then doing the result of that thinking isn’t actually new, and the other is that the particular style in which the EA community pursues that idea (looking for interventions with robust academic evidence of efficacy, and then supporting organizations implementing those interventions that accountably have a high amount of intervention per marginal dollar) is novel, but mostly because the cultural background for it seeming possible as an option at all is new.
To the first point, I’ll just list Ethical Culture, the Methodists, John Stuart Mill’s involvement with the East India Company, communists, Jesuits, and maybe some empires. I could go into more detail, but doing so would require more research than I want to do tonight.
To the second point, I don’t think that anything resembling modern academic social science existed until relatively recently (around the 1890s?), and so prior to that there was nothing resembling peer-reviewed academic evidence about the efficacy of an intervention.
Giving them time to develop methods and be interrupted by two world wars, we would find that “evidence” was not actually developed until fairly recently, and that prior to that people had reasons for thinking that their ideas are likely to work (and maybe even be the most effective plans), but that those reasons would not constitute well-supported evidence in the sense used by the current EA community.
Also the internet makes it much easier for people with relatively rare opinions to find each other, and enables much more transparency much more easily than was possible prior to it.
I think that the wisdom of nature prior would say that we shouldn’t expect blasting a neurotransmitter pathway to be evolutionarily adaptive on average. If we know why something wouldn’t be adaptive, then it seems like it doesn’t apply. This prior would argue against claims like “X increases human capital”, but not claims like “X increases altruism”, since there’s a clear mechanism whereby being much more altruistic than normal is bad for inclusive genetic fitness.
I would worry about this more if the OP were referring to a specific intervention rather than a class of interventions. I think that the concern about being good on longterm and shortterm perspectives is reasonable, though there is a proposed mechanism (healing emotional blocks) that is related to both.
Normal drug discovery seems to be based off of coming up with hypotheses, then testing many chemicals to find statistically significant effects. In contrast, these trials are investigating chemicals that people are already taking for their effects. Running many trials then continuing the investigations that find significance is a good way to generate false positives, but that doesn’t seem to be the case here, and I would be surprised to find zero effect (as opposed to shorter or different effects) if it were investigated more thoroughly.
I also think that improving human capital is important, and am not convinced that this is a clear and unambiguous winner for that goal. I’m curious about what evidence would make you more optimistic about the possibility of large improvements to human capital.