What's in it for him? Likely loss of his primary revenue stream and control over the project's destiny? It doesn't seem like a compelling move.
Besides, going open source doesn't mean that you lose the keys to the kingdom. There are plenty of open source projects that make money; Linus is gainfully employed and he gives everything away. I'm sure there are ways to monetize ST beyond charging for binaries.
Use Kickstarter to open source it and you both cash in and do the community a favour.