Even if it is stable, it may have accreted many features over that time such that the code is now difficult to modify. Being able to leverage standard c++ stuff

Even if it is stable, it may have accreted many features over that time such that the code is now difficult to modify. Being able to leverage standard c++ stuff could be quite valuable, and could even result in less code (for them to maintain). They appear to be doing it very gradually, where the first step is probably little more than simply compiling the existing code as c++ instead of c.