I had a very successful contracting career for 9 years which started off using C and X-Windows Motif - developed on Sun workstations and deployed to Alpha/OpenVMS servers. This was at British Gas the application was a graphical network analysis tool with Fortran analysis engine to determine flows and pressures in local gas pipe networks.
After that I moved to the dark side of Java which made developing very large programs much easier - a safer option but not with the elegance of C.
A couple of years ago I was contacted by someone still working on that application. When I left in 1998 it was due to be replaced with software from an external company British Gas had bought. I didn’t see how that could ever happen given how bespoke it was, and clearly I was proved right in the end!