My prediction is that C will die off as a language long before Fortran and COBOL. What keeps a language like Fortran alive is that it is used in an application niche where rewriting is done at best ship-of-Theseus style, and the friction of using a new language for a component far outweighs the benefits of doing so (weather models are a good example here). For COBOL, it remains in applications where the cost of a switch or rewrite includes a low risk of catastrophic, and therefore expensive, failure (see Southwest Airlines for a very public and recent example of such a failure).
C does have a similar kind of niche at first glance: systems programming is of course conservative, rewriting everything is unlikely, and of course, C is the language used for ABI. Except on closer inspection, that moat is remarkably shallow. Being the language of ABI means that every competitor language has some way to speak C guaranteed, so the friction of rewriting systems software Ship-of-Theseus-style is much lower (though still nonzero). Systems software is rewritten from scratch on a much higher cadence: in the last 20 years or so, most of the userspace system glue for Linux has been replaced (e.g., systemd, iproute2, pulseaudio, wayland). And we've learned over the past few decades that there's no practical way to fix C's fundamental unsafety issues with software engineering practices, and C's committee is too conservative to consider retrofitting the necessary features to be able to fix unsafety at a language level (to say nothing of getting people to use it).
There is already a small clutch of languages that can serve C's niches that don't have the same fundamental unsafety issues, and right now, we're sort of at an experimental stage of system software trying them out. It's not unreasonable to believe that within a decade or so, one or more of these languages would be considered a standard, safe choice for implementing new systems software--and the use of C in new projects will start dropping. At some point, the proliferation of non-C systems projects will make people point out that having these components talk through C's ABI is too limited in functionality, and a system will change its ABI from C to some other language. And once it is no longer the language of ABI, C will lack its moat that keeps it alive, and it will start dying, though its death will be a slow, agonizing death.