ABI isn't just what rewriting code. Even if you rewrite the code, you still have to test it, certify it, and so on. Because C++ has an ABI, I mix code that is already built with new code and only recertify my new code.

It is a huge pain, and quite honestly, most of the reasons I have to maintain ABI are political and thus not up for a technical discussion, but I still have to do it.

The compiler vendors care. It is also the reason why C++ never actually had a GC, even if the standard had it specified (and removed it with C++23).