Linking COBOL and Fortran against C is actually one of the easier things to do. So with these multi-million line systems, often new features are written in C++, minor minor minor bugfixes in the original language, and general maintenance is a toss-up if it's a re-write in C++ or just fix it in the old language. Depends on how fast they need it.

I did some Pre66 Fortran work for a bank, and that was basically the idea. They were in the process of converting internal assumptions of the program from a very particular older mainframe type to a mix of Z/OS and Solaris, and the Fortran/COBOL into something they could continue working with.

Some of the code was very specifically marked as "not for C++", because modern Fortran (2003 at the time) was better for some specific number-based things. But that code becomes a library rather than part of the original monolithic structure. And modern Fortran is actually quite a nice low-level language to play with.

It was actually easier than expected - when the original program was written, and for about the next twenty years following, the entire program and changes were very well specified in multi-thousand page specifications. The newer code was harder to keep track of, rather than the oldest, because at the oldest those documents were 99% accurate, even if you never trusted them to be because of the intervening time.