Largely true, but C/C++ don't depend on compiler extensions they are standardized by ISO.
They leave so much undefined, and most programs don't strictly follow the standard.
A hypercentralization would be GCC extensions and quirks dictating the C/C++ standard
That already happens, with GNU extensions being standardized.
and everything else being a poorer replica of GCC
Again, that already happens.
with standard replaced by 'whatever GCC compiles is right".
Look at several of Linus Torvald's rants about how the standard doesn't matter and how GCC should just read his mind and do what makes sense.