The ones that came on in the 60s/70s were mostly retiring out before the Shuttle Program (and our own ability to launch humans) was ended without a funded follow-on program. It's the ones that came on in the 80s/90s that were dumped, unceremoniously, when Atlantis came to "Wheels Stop" on STS-135.
There are a few, including some of my old colleagues, who are still there and more than capable of doing this -- but yes, it's the loss of institutional knowledge (even if it's been documented) and perhaps the ability to train of the next generation of Manned Spaceflight Flight Controllers that may be lost forever.
It will require YEARS to really build that back.
"Flying in circles" with ISS has not done the MCC teams any favors. When you're in a dynamic phase or doing something you've never done before requires a completely different set of skills and experience. There's a lot of emergency thinking and reaction skills that have been lost... or at least dulled.
I've been critical of the SpaceX teams for taking too lax an attitude in their "MCC". Once they start flying *people*, a different and more focused approach is going to be required, or people will die (i.e., NO FUCKING GLASS FISHBOWL AND FAWNING/CHEERING FANBOY AUDIENCE).
While I love the idea of a nationally-focused Manned Spaceflight Exploration Plan™, the phrase "Urgency must be our watchword," goes against EVERYTHING in the Flight Controller's Creed:
Pay special attention to #2... "ultimate consequences" means something. Nowhere in that Creed is the phrase "go faster" found. In fact, the opposite ("taking a stand when we must") is one of the most vital phrases (and one of the failures in both the Challenger and Columbia accidents).
Getting "GO FEVER" will result in people being killed. And that could be the end of the US Manned Space Program.