What is an expendable player? In my mind it's a player that has proven they can play at the NFL level, but that the team reluctantly has kept around until they can find a better option.
Depth at a particular position can make veterans expendable. Younger players might be expendable if they stopped improving and are being pushed down the depth chart by other players.
Players that can't stay healthy might be expendable too as the team has likely found a suitable replacement while they are waiting for them to heal.
There are all kinds if reasons a player might be expendable, and no team is without a few of them.