Well as a fellow old fart.. I can say that the mainframes are going away as fast as people hit retirement age. Its as if the mainframe is the gated retirement community. There aren't a lot of them and they are expensive when they go down so only the experienced people are allowed on them.. and the new people play with this Linux stuff... and all the paycheck software that was on the mainframe is moved over to the Linux systems for what 1 month of support from IBM etc cost.
The same is the way for any language.. yes it is ubiquitous and will be around for a long time, but eventually it becomes a Fortran or Cobol. The community is a small set of people who used it in its heyday and a smaller crowd of people who are "accepted" into the Secret Brotherhood. And eventually the decline becomes terminal where only people in a certain set ever really know the language anymore.
One could say this is natural course of events for computer languages.. they are born, grow, become mainstream, retire, and then spend their final years sitting on their front porch yelling at the kids to get off their lawn. Some sadly spend their last years in a nursing home with tubes stuck up every orifice because the kids are living off their pension checks.