The backbone is a minority of the bones in the body too, but without it you're screwed.

My previous job was at one of the largest investment companies in the world, if the COBOL went away the company would collapse.

While the argument is generally correct, Fortran and COBOL aren't in the same league. Fortran is still being upgraded and has some use cases that are hard to achieve in another language. I've seen the Rust subreddit recommending use of old Fortran code with Rust, rather than rewriting it in Rust. Unlike most other languages like C++ and Rust, Fortran kept its scope mostly limited to numerical processing - making it still one of the best solutions to code highly parallel numerical algorithms in.

Fortran isn't surviving just because nobody wants to rewrite Fortran code. Unless you're talking about Fortran77 code.