Stapled may not be as bad of a thing as "fenced." Ask any former Republic pilot from the 80s now flying with NWA. My uncle wasn't able to upgrade past the 9 (Republic's original equipment) for like 10 years or so. And he was unable to bid 74 captain even though he is senior to left-seaters...

Red list, green list (or blue list) fences. Not sure what it was called.