"This is why building an MVP, and validated learning are so important. Before we get too carried away, we have to find out if the market wants what we're building." Spending more time on an idea without getting substantial success may lead to 2 things... 1. Either you loose interest and quit where the product actually might have been valuable if it would have seen the light of the day. 2. You take it to your heart that even if there is no real market-fit you still keep working on it.

Either of them are dangerous for an entrepreneur. "Fail fast fail often" is the way to go.

"Speed is the essence of war" - Sun Tzu (The Art of War). We can safely say that it applies to startups too.