I think those are a couple different trajectories there. 37Signals/FogCreek both have modestly successful sidelines attached to massively successful software businesses. DHH is not going home at night thinking "Bwahaha, forget Basecamp, another book and I'll have to rent Lichtenstein to park my fleet of sports cars." Amy & Thomas have gone from consulting to info products to SaaS (concurrent with info products/training) and I will eat my hat if the SaaS doesn't eventually swallow the rest of their business because the economics of it are just so freaking compelling. I don't have a good guesstimate of Rob's numbers.

For my part, the trajectory was different: modestly successful product (and ancillary blogging/community participation) lead to lucrative consulting opportunities and gave me enough runway to do a SaaS which will probably eventually swallow any availability for consulting (like it did for 37Signals, FogCreek, etc).