Even if you accept that Musk is currently the best possible head of SpaceX (which in my opinion is a very strong claim), he probably won't always and forever be the best possible head of SpaceX.

Musk's strength is in making bold plays, assembling teams together to make those plays, and building hype around those bold plays. Those skills were certainly critical for both SpaceX and Tesla in the early days, anyone denying that value is either delusional or pushing a revisionist history. But as companies grow large and mature, their needs change. Is Musk the leader you want when you need to do the boring work of making your infrastructure more robust? Would Musk even want that job?