I'd like to say here that having some perspective can prevent certain kinds of mistake. I think that's correct.
I'd be on surer ground if I point out that a person entirely satisfied with their professional perspective isn't going to improve past a certain asymptote that they themselves are unable to perceive.
But honestly? I just find self-congratulation tremendously embarrassing. I don't know if it's inherited or a self-deprecating theme present in Australian culture or just plain old jealousy. Or some combination of the above.
And some of that pruning is good and necessary. Otherwise, bad ideas would proliferate and take air and light from the good ideas. But often when I read HN's comments, the field just looks like scorched earth. A complete, overpowering negativity everywhere. And I think then the pruning has lost its purpose, and is a bad thing.
I suppose, I would like that whenever an idea was dismissed it would be by pointing to a better idea in the vicinity which could then be thought about or worked on instead. That way the true purpose of criticism and pruning would be clear: To not let the bad ideas take resources from the good ones.
With regards to self-congratulatory behavior in general: well I suppose I understand you. I've been brought up that way too, as has perhaps most people. Still, I recall Russel's own adage:
“The whole problem with the world is that fools and fanatics are always so certain of themselves, but wiser men so full of doubts.”
I don't know the author of the OP, but I do certainly think that negative attitudes create even more doubts in the wise, while fools and fanatics likely are innately immune. Therefore, I don't think that unreserved dismissals usually will do anything to people's character, except for strengthening those traits you really want to see less of, in the wise.
And with three or four, it's probably something inbetween.
What is the problem with being self-congratulatory? What is wrong with focusing on the positive and thinking that your hobby and life's work is worthwhile? Would anything be created if people were constantly doubting themselves and their creations? Would anything be better then?
What harm would there be in people hoping to be the next woz? And with the low base rate of garage projects going anywhere, which we are all painfully aware of, don't you think it's better to give encouragement and hope. I do, and I think a dismissal such as yours is harmful to the greater cause of creating interesting ideas, programs and thoughts.