GCC will probably never be replaced. The current C++ standard is so monstrously complicated that implementing a full compiler would be an enormous task. But it doesn't matter, because eventually few or no new projects will be done in C++. Consider the fate of COBOL, which was once a shiny new language, a real step up from coding business application in assembler. It's now a tired old has-been, used only for patching up legacy systems.

As a system ages, it becomes more and more complicated because it is serving more and more purposes. Accordingly making upgrades while preserving backward compatibility becomes harder and harder. I remember hearing about an old AT&T switch with software so intricate that in the end it required one meeting for every line of code changed. And so progress grinds to a halt.

At some point, you either break backward compatibility or face stagnation. Careful modularization and upgrades can push back the boundary, but not forever. Even good practices, good people, and a good organization can only tame so much complexity.