Once a language no longer has a developer pipeline, it becomes a language with no future. No matter how important and widely used. You wind up with jobs begging for qualified developers. Given how systems last, the language may survive indefinitely. As nobody dares migrate projects off of it. But employers will struggle more and more to find employees.
The first language that this happened to was COBOL. There are still a lot of COBOL systems out there. And a lot of COBOL jobs. But nobody wants to go into COBOL for fairly obvious reasons.