Libstdc++ is normally distributed with the compiler, so it will be updated as required by the standard.

Regarding the libc and other headers, modules will still fullly support include files, so that won't be an issue. You should be able to include headers and export them as modules.

Kernel/system headers are C, or part of a compiler SDK.

libc and libstdc++ are part of the compiler runtime, ie. will be updated together with it.

Just kind of.

Kernel headers are only an issue on free UNIX like OSes, on other OS what matters is what the SDK provides.

So at least in what concerns Apple, Google, Microsoft and every other commercial vendor OSes, don't expect a big issue having their SDKs updated when they start supporting modules.

The problem is getting the BSD, Linux and open source libraries without a specific agenda to adopt them.

Clang uses "module maps" to turn existing header-based libraries into modules. So, theoretically, your system compiler could include a module map for all of the system headers. I don't know if module maps are going to be part of the standard, though.