That's only a good idea if the veteran actively prevents the situation from getting better, e.g. by being possesive and contrarian, trives on being the hero etc

That's only a good idea if the veteran actively prevents the situation from getting better, e.g. by being possesive and contrarian, trives on being the hero etc. This does tend to happen a lot in these situations. The team will also need to have the skill to actually do better. If the veteran is not sabotaging things firing only throws away the most valuable source of knowledge the team has.