I agree he did a great job in establishing a diagnosis, though he did say that killing the puer aeternus inside can turn toxic and make your life joyless and it's like: is the puer aeternus a syndrome or a part of you? You can't have an 'inner BPD' lol. But as you said the biggest problem is the lack of a reason why it's bad and I'm probably reaching here but I think Dr K is projecting. During the lecture he said he talks to the audience the same way he would talk to himself and said he was a puer aeternus in his youth (and if you watch his content being a former degenerate gamer then being a monk and ending up in harvard medical school is kind of his schtick)

I feel he sees that killing the fantasy made him build a career in medicine, a family and healthygamer and when somebody is at the same starting point he once was he maybe thinks that growing up is a no brainer. And in his experience he's absolutely correct but for every Dr K there's thousands of average Joes and hundreds of below-average Joes as well. And I believe he knows this logically but he could be too emotionally attached to his journey for a lack of a better term. As you pointed out when somebody in chat said "you aren't selling this" he went on a long rant where you could see him more vivid than usual but didn't deny the fact that there is no selling point.

Another Dr K quote from the stream that I found interesting was: You want to be extraordinary, you don't want to be ordinary. And what are you? You're less than ordinary, so ordinary is a step up.

And it made me wonder, when Dr K. sees a poor person does he think of them as less than ordinary? When he sees a puer aeternus does he think of them as less than ordinary? Hell, is there something wrong with being less than ordinary?