the guy asked for feedback on his product, not his life decisions. to make the leap is presumptuous, pompous, and, yes, unnecessarily negative. there are polite ways of giving negative feedback, for instance you might have said: "hey thanks for posting. looks like you've done a good job putting it together. i worry that there isn't a market for it, though, given all the competition that currently exists and people's general inability to monetize these sorts of products. maybe use this as a jumping off point for something else?" instead, your original comment is full of vitriol and, in one fell swoop, you've managed to put down the creators of "thousands of project management apps, hour tracker apps, wannabe social networks, twitter sentiment tools, accounting apps, photo filter apps, a/b testing websites, etc." you, sir, must surely be without flaw to make such broad and sweeping generalizations about your fellow community members.
it continues to amaze how certain members of this community cannot recognize the vast chasm of constructive criticism that lies between the extremes of '[telling] him that the idea is awesome and that he will become a millionaire' and telling him he has wasted his time working on an idea that "isn't even a real idea" (and we won't even address the absurdity of that last sentence...).