It's not actually the choice of the president whether a single issue is the only issue, regardless of how much he might want that to be the case. If elected, he would have to run the executive branch, which means he would need to select a cabinet, unless he is planning on just hanging on to the previous guy's choices until he has finished his quest. We're going to judge him on his perceived ability to do that, as well as everything else a president has to do.
Further, if he is perceived as competent enough to do all that, why would I vote to hire him as chief executive of the nation when he has stated he is going to quit soon after he is elected? His running mate will have be chosen far more carefully than normal. Also, why would I be willing to leave the decision about the replacement vice president to a bunch of people in Washington? By pledging to quit, he is effectively eliminating our direct voice on half of the ticket.
Mr. Lessig, if you want a chance at my vote you have to run for real and commit to serving your full term to the best of your abilities. I hope you do—I think you are up to the task.