If you need an explanation as to why a guy would be unhappy because people are upgrading (for lack of a better term) out of seniority. Then you haven't experienced a real seniority system.
These people are "taking" (if you like that word better) a position from an S/O who have put in years to get. They're no better than the "champs" who came to CX during the recruitment ban.
They'll be the first to stab YOU in the back if/when necessary.