Clearly the idea (or the solution) is important - my argument is that knowing the right solution day one isn't very important. Airbnb and Twitch are great examples of this.
You're extrapolating from the winners and applying that to the masses. Read up on their history; Airbnb and Twitch almost died multiple times and that's not a good model for any startup. Even startups that actually start with a good idea fail frequently.