So many people hear things similar to this, nod in agreement and then proceed to make the same mistake! I don't get it. Somehow they think what they are doing is not the same thing. I guess this is the reason most good advice have no positive impact.
It's hard to evaluate your own work objectively. There's a bunch of psychology around sunk costs, etc. that don't affect you when you're looking at someone else's project.
A friend of mine is doing this right now. The project involves a whole pile of electronics, a card system and a bunch of other things, he's already working out how to make millions of the things but he doesn't have a customer yet.
The scary thing is he is hugely successful at most of the things he does. It's going to be interesting to see how it comes out.
I learned it would have been much better to rush out an early prototype than to spend 7 months developing apolished product that no one would use.