I think both are, long term (think FORTRAN where it’s not particularly popular but a lot of existing code is maintained and not rewritten).
C++ is actually in a slightly better spot ironically because it’s harder to integrate with. If you have a C program you can pretty easily start replacing parts with Rust. You can’t do the same with C++ which insulates it better in that sense.
Reports of Fortran's death (latest standard 2018) are greatly exaggerated (much like C). It's receded to a niche, but it's still a very important niche (numerical, HPC). Hopefully, the development of a new Fortran front end for LLVM (from PGI/Nvidia?) pans out, as this would fill a gap in LLVM's offerings, and provide more competition for ifort and gfortran.
C++ is actually in a slightly better spot ironically because it’s harder to integrate with. If you have a C program you can pretty easily start replacing parts with Rust. You can’t do the same with C++ which insulates it better in that sense.