C99 is a pretty tiny (but welcome) incremental improvement on C. It's also almost entirely orthogonal to C++'s additions. Calling C99 a C++ replacement is pushing it, so you'll need to elaborate on why you think so. I'd be curious about an instance of something you can do better (let alone fundamentally better) in C99 than C++. I can think of plenty of examples of the opposite.
C++ sometimes makes me want to tear my hair out, sure[1], but when using pure C(99), I find myself thinking, "this code would be so much more compact in C++."
[1] This is usually because either I or someone else tried to be too clever. Otherwise it's because of something C++ does badly that neither C89 nor C99 do at all.