5 million lines really isn't that much in the grand scheme of things.

I believe the company I work for is writing just under 500K lines of COBOL code per year... now... how much of that is "new" vs extending vs maintaining is a good question. It is very difficult to measure those things in our environment. If you were to ask the 60 - 75 mainframe programmers we employ, I'm sure they would answer that COBOL is very much alive, and no matter how hard you try to kill it, 40 years of system code is just not going away any time soon (depending on your definition of "dead"). Especially as we hire a good amount of "new to us" people to maintain the system.

I think individual companies should define a language as "dead" based on the number of new people needed to maintain the systems. As the number approaches less than 5%(?) of your replacement hires, have you effectively "killed" the language? At the very least it is on life support, and a decision needs to be made about its future. (A grim analogy, I know).