The DCNS concept was provided only as an illustration of what is possible with 2015 technology. Power density in battery technology has been increasing at the rate of 50%/decade for the last thirty years or so. Improvements in AIP technology are slower, I expect.

With issues like this and significant operational limits the question, for me, would be who has a requirement that this kind of SSB is the answer for?!!

This seems to be the sticking point. If you are already committed to supporting SSNs, from naval reactor design, to shore support facilities, the investment required for a nuclear-powered ballistic missile submarine is correspondingly reduced.

Nonetheless the obvious candidate nations would be those looking to field nuclear weapons on conventional submarines anyway: Israel and Pakistan. In the event that they decided to pursue a nuclear deterrent, South Korea and/or Japan could be interested in such a platform as well.