A cofounder is most critical a factor in the early stages of a business. Apple and Microsoft were already established companies to a larger degree when the cofounders left
Sure. Still, what I said is on target: Gates, Jobs, etc. did the big, huge growth just from their own brains. There's no good reason to believe that they could not have done all of the success, including the first days, essentially alone also.
Really, for the "early stages of a business", that is JUST what Jobs faced at Next, Pixar, and back at Apple. What he and Woz did for the Apple II was nearly irrelevant except it did leave Jobs with the cash from his sale of all his Apple stock except one shart. So, we're not talking the value of a cofounder but the value of a founder with cash!
And for Gates, as MS/DOS took off, Microsoft was just awash in cash. About then, Allen left. So, the real building of Microsoft was due to Gates and cash. It's not the least bit clear that Gates ever really needed Allen as a cofounder. Similarly for Jobs and Woz.
Early stages of a business in this context is way earlier than what you are referring to. We are talking the stage that YC gets involved: usually just the founders, working out of their apartment / garage. That is not, as you argue, the same situation that jobs faced at those other ventures.
