My feeling is that if Zig drops C++ support, there seems to be enough momentum on the C++-dependent users, that they will just fork Zig...

Can't LLVM be optional, and have unit tests guarantee that it works as before?

The above, or - as mentioned elsewhere, implement a C++ compiler in Zig: quickly, and before making any breaking changes - quite a tall order.

Or bring into Zig an existing pure-C, C++ compiler.

...TinyC/libtcc? PCC? LCC? (wild guesses)