I generally develop custom child themes for my clients - I've done about 50 of these. Usually it's on a flat rate, and then I am hired back occasionally by the hour if further customizations are needed. Many themes get updated for a time (which is why I make them safe with child themes), and then eventually, the updates stop. This could be true for a free theme as well as a premium theme. With premium themes, what happens sometimes is you develop something as a child theme of Thesis, or Custom Community Pro, and then suddenly, they come out with Thesis 2 or Custom Community 2 and announce they will no longer update Thesis 1 or Custom Community 1. (And in those cases, 2.0 is a completely different animal and you can't just switch the site over without redeveloping it). So, what happens? Can you just leave it that way forever, with no further theme updates? Continuing to update WordPress itself, but with no more theme updates?