They'll do the same thing the ECMOs are doing as their airplane has gone away...
Met a former student of mine last year...He's USMC ECMO and is now flying as a EC-130 backseater under some type of exchange program. He's still a Marine and is still flying for the AF. I've heard a few USMC ECMOs are now doing UAVs.
But no clear path for USMC NFOs once the planes are gone.