Just before I left JPL, the SPICE team had announced a plan for a complete rewrite in C++. The SPICE team is exceptionally small (especially considereing how widely used and impactful the software has been). IIRC since the team’s inception, to the time I left, it has been about 4-5 people. Their primary goal has always been stability and correctness over speed. Recalling a conversation with Boris, they have something like 2.5 million lines of test code. So it would take some time to port over. The codebase is probably the most documented I have ever seen, every mathematical deduction is described in great detail.
That being said, as someone who has integrated CSPICE into several C++ and python projects, the modernization would be a very welcome change. The current arch depends far too much on global state, and none of it is threadsafe.