Well, using right back as an example.
Coleman is 29 and Kenny is 20.
At the end of next season, Coleman will be 30 and Kenny 22.
If Kenny has taken Colemans place at the end of next season then the club has a decision....
Should they keep the 2nd choice right back whose on a big salary, or should they sign a young player with high potential to push Kenny?
Also likely, would Coleman ask to leave for game time or if Kenny isn't getting games would he want to leave...
In each of these scenarios the short term vision of keeping old players too long isn't the ideal choice. After 30 their values plummet as well.