There's an abundance of people too. You wouldn't say people don't matter.
You can teach the importance of execution by emphasizing execution.E.g. two teams with the same good idea, one executed and the other one didn't. You don't need to devalue other factors in order to make a point.
People that don't execute don't matter. The idea of an "idea" is too abstract to have this argument really. If an idea is as long as a sentence with no further in depth exploration, it really doesn't matter. If it is becoming more of a plan then you might say it matters (which would be going down the path of executing an idea).
You can teach the importance of execution by emphasizing execution.E.g. two teams with the same good idea, one executed and the other one didn't. You don't need to devalue other factors in order to make a point.