It's probably not going to be a rewrite of existing systems, but a switch to new systems written in safer languages while phasing out the old C based systems.
On the server: unikernels. On the client: new mobile platforms, like OSes based mostly on web technologies (which while not quite being there yet are getting more and more powerful).
C/C++ is not only a poor choice in regards to security, but also to develop for hardware with hundreds or thousands of cores, and all hardware platforms are moving in that direction. Yes there are of course ways to deal with that in C/C++, but other languages are much better at this.
Within just a decade or two I can see a fundamental shift away from the old computing paradigm, at least for the majority of systems in mainstream use.
