You've already the obvious answer that ideas are worthless and execution is everything. The more nuanced version of that is that every idea is a bad idea initially.

Even good ideas are bad ideas to begin with. As soon as you start trying to work out your idea, you run into all kinds of problems that need to be solved.

And that's fine. That's really what any creative who produces finished products does. They refine a superficially good first idea over and over until it is an actually good idea that results in a finished product.

There's countless good ideas that turned into shit final products because people screwed up that process. And there's plenty of absolutely fantastic products that started out as a very unremarkable ideas that got polished to perfection.

But someone has to do all that work and that person is the real artist. Trying to sell an idea is like trying to get someone to pay fine art prices for a block of stone with the argument that the sculpture is in there somewhere.