I am out of the loop, but I thought Loyola, Drake, Missouri State, and SIU were retaining almost all of their best players. Also SIU is supposed to be getting one of their best players back from injury next year...
Those 4 teams have been hurt less by transfers than some other MVC teams. But all 4 have lost players to transfer.
Drake lost a couple players, including Joseph Yesufu, their best player down the stretch.
SIU has lost 2 players so far, a freshman guard (Eric Butler), and a grad-transfer (Jakolby Long), neither of whom ever played much. But they were a 9th place team in 2020-21.
Missouri State lost Jared Ridder, one of their starters who averaged 25+ minutes per game.
Loyola lost 3 players, but none were among their top scorers (backup center Franklin Agunanne, sophomore guard Cooper Kaifes, and freshman guard Baylor Hebb). But as you noted, the loss of their coach could have a significant effect.
The SIU player you are referring to is juco forward J.D. Muila, who missed the 2020-21 season with a knee injury and surgery. It's hard to know how much difference he would have made or will make next year after returning from major knee surgery. He was not highly recruited out of juco. In fact, despite not committing until very late (April, 2020) in the 2020 recruiting cycle, SIU was the only offer that he had. Muila's sophomore season at Indian Hills CC he started and played a lot, but only averaged
6.8 ppg. IMO, he will probably be a backup to Kyler Filewich, their starting center in 2020-21 who looked pretty good in his freshman year.