My feeling is that Joel is really just getting tired of his comfortable, but small, operation. If he keeps doing what he's been doing he'll probably be relatively small forever.
37 signals is in a similar situation. They're really small relative to what's possible. As far as I can tell they're on course to be small forever, in terms of revenue.
You already have a good product. You know there's a market. You know people are willing to buy it. Now what do you do if you decide you want to accelerate your growth a lot?