Several things. One, Valve is a privately held company, and I believe gaben holds a majority of it. Two, In UT's scenario, he leaves the company without retaining a voice in it (see point one why this is unlikely). Three, in UT's scenario, he leaves the company in the hands of trustees that are not accountable to him in one way or another. Four, those trustees lack common sense (given that holding a piece of a private company that enjoys high regard in its field, and has a steady source of revenue is certainly preferable to holding a piece of a public company, for the simple reason that being a shareholder in a public company means having to deal with the other shareholders and their lawyers all the time). Five, it assumes that there is a scenario where doing an IPO makes sense for Valve as a business (Hint: Unless you're really short on cash, doing an IPO is a bad idea for the reasons discussed above re: loss of control).