i don't think there's much to this.
shares are up so he sells. Also, as the thing says, it's for share diversification. also the more he sells, the less of a monopolist he seems.
Plus, even if Microsoft survives till gates retires, their share price will have fallen quite a bit by then.
Even if the masses do not see Microsoft are going to slowly tip over the precipice, i think gates has further sight with regard to his own money than many around him.
however as i say, in this case i think there's not much in it.