With such a codebase, once it is settled to a certain point, you stop adding things. You write new things, and carefully interface with the old stuff.

Imagine a bank. Their software core is usually neolithic, i.e. written in COBOL or even worse, FORTRAN. You don't add the "online banking" or "web client" interface in the original language. You add them in something more contemporary, which interfaces to the neolithic core via files, pipes, libraries, whatever, and translate it into a frontend as needed. As long as the core works, nobody needs to touch it.

I wrote a web app for a client back in the late '90s that is still in (heavy) use at the company. It was actually a "Classic ASP" app and they kept one old PC around to act as the server for it for a couple of decades (they eventually replaced that with a virtual machine and the app is still going). The output is straight HTML + CSS so they've never had any problems using it with progressively more modern browsers. Ironically, this app is a front end sitting atop an unbelievably clunky mainframe application that dates to the 1970s, so my app's continued existence means that mainframe application is still running as well.