He leaves after the 09-10 season. In 09-10, the Cavs had the #1 record in the NBA, with 61 wins. Who was their second best player? Mo Williams? Zydrunas Ilgauskas? He leaves, they go to 19 wins.
His last season in Cleveland he made it to the Finals and had 50 wins. 17ppg Kevin Love was their best player. The next year, they won 19 games.
Your argument is that MJ's "abandoned" Chicago teams still had their second best players. Both times LeBron left Cleveland, Cleveland still had their second best player on the roster. So, when Jordan left, the Bulls (with their second best player) made it to the ECF. When LeBron left, the Cavs (with their second best players) went to 19 wins.