I agree with you, but ..

Through direct experience, I do not think 2 or 3 good sessions and you're done is really true. Maybe with experience even that goes - you learned what you needed to learn and there's nothing more to it.

But IMHO psilocin is a strong anti-inflammatory which may in and of itself be some (not all) of the mechanism for alleviating depression (the link between inflammation and depression is strong). That will almost never be lasting.

So we may be in the case where the real breakthroughs are, as you say, in the first two or three sessions. I think most people who took mushrooms out of desperation would agree with that - by the end of the third session many of the things they needed to accept are, in fact, front of mind for awhile. But ongoing maintenance is probably useful.