It's not. Say your team is: Conference player; Conference player, Conference player, Conference player, Conference player; Conference player, Conference player, Conference player, Conference player; Suarez, Messi.
Summer rolls around and Suarez and Messi leave. With the money you earn you go out and make some new signings and your new team looks like:
Conference player; League 1 player, League 1 player, Conference player, Conference player; Conference player, Conference player, League 1 player, Conference player; Conference player, Conference player.
Your team has has improved in three positions, but the team as a whole will still be significantly worse.
It's mental and extreme, I know.