Throughout my career, I’ve heard that lead designers should be the visionaries for their projects. That statement contains truth, but it doesn’t contain the whole picture. The lead designer for any project must know what they wish to make, yes. Heck, the lead designer should know what they’d want to make if they had an ideal set of development circumstances.
Knowing what you’d like to make in an ideal world, however, isn’t the same as knowing what you can make in the real world.