A lot of players are incapable of improving. You see them every season, at clubs up and down the country, players who are deemed not good enough and moved on.
Nobody said David Moyes was a crap manager when Tony Hibbert reached his zenith, they just accepted that there was no more ability to drag out of him. It's not uncommon. Managers aren't miracle workers. I reckon a good 70% of players in this league will never be good enough to reach elite level, and no amount of coaching will make them that good.