Basically, the team didn't plan on working on ESF forever. When they started, Half-Life 2 was out of sight, out of mind. The team has come a long way since the early alphas. And to start all over again on the new engine is something the team doesn't want to endeavour.
Right now, I think the team is working towards a finished ESF. Whether that be 1.3, or 1.3.X with some bug fixes. I think the team wants to finally be able to say "we're finished". And move on to new projects whether they be programming related or not. ESF is not a life-long project, it's a short-term hobby, and the end isn't too far away.
But do not worry, I am more then certain that there will be DBZ mods out for the source engine. You just have to wait for them.