"Last week I read Bill Gates will have sold all of his Microsoft stock by the year 2018 through a series of planned transactions that began many moons ago. I was dismayed by this. Why would he sell all of his stock?"

I don't know why smart people keep repeating this over and over again. Who says Bill has to sell all of his stock? His stock selling schedule does not go to 2018. Yes, if he continues to sell at the current rate through 2018 all of his stock will be gone, but that's an unrealistic extrapolation. Gates has never said he plans to sell all of his stock by X date. He holds $63 billion outside of Microsoft. I'd argue it's at least as likely that Gates will hold onto a multi-billion dollar stake, and that his goal is to limit risk by not having such a singular large position in any one investment. His goal being to maximize the amount of money he can put to use in the foundation, and minimize the chance of a big portfolio loss.

It's also worth noting that by 2018, he will have been sitting on that stock for roughly 43 years! That's an extraordinary length of time for a founder or any owner of any stock to hold.