The paper you link does not mention anything about the need to try every potential treatment (and no treatment) on every person in a study, and it does not ment

The paper you link does not mention anything about the need to try every potential treatment (and no treatment) on every person in a study, and it does not mention anything about any "inherent bias" in RCTs -- these were the two claims you made -- so I'm not quite sure why you bring it up. Given that you have brought it up: I like Deaton's work a lot and know the arguments well, but you have to take into account that in this paper they are trying to provide some counterweight to a hype cycle that suddenly led a bunch of social scientists to argue that observational studies are stupid and should just be gotten rid of entirely and replaced by randomized trials, which for many reasons is not a good idea. That hardly implies that we can never know anything about whether psychotherapy works until we can simulate the brain, that's just a totally bizarre conclusion to draw from that paper.