I don't think a license has to expire if another dubbing studio wants to dub an anime. Aniplex and Funimation has done it a few times, they more or less just work together, I think...if they ever choice so.
Bad example! Your example is dealing with two separate licenses.
For example, Bacanno comes into mind. Funimation acquired the DVD rights, while Aniplex USA acquired the Blu-ray rights. The licenses between the two titles was split. Since Aniplex Japan owns the franchise, Aniplex USA had access to all of the materials that Funimation made for the show (dub, subs, extras).
Dubbing studios don't acquire anime titles. They only do contract work for the anime studios that own the license to the show.
There's no way in heck that Sentai is going to sub-license to another company to get a sub only title dubbed. Sentai dubs everything that they believe will benefit from it. If a sub only license doesn't get a dub, then it's a good indication that the title just broke even or bombed.