Obviously it's trickier to develop a multiplayer game, but the economics seem so compelling; in a 15 year career, he's never broken out into multiplayer? Or is there something I'm missing about those economics?
I bought one of the games recently and emailed him a bunch of usability/interface suggestions. He thanked me but mentioned that he's making a living so somebody must be enjoying the games. He's got a point.
It's a labor of love. I'm sure there are a million things he could do to add the potential to make more money (like an investment in better art or another programmer), but it looks like he's not interested in that kind of risk.
It doesn't seem like an ambitious plan, but he seems happy so what's wrong with not trying to experiment with business models and whatnot?