Thanks to the transfer portal it doesn't matter what players a team loses from one year to the next. What matters is what players the coaching staff is able to bring in. What WKU did a few years ago is a great example of how a team can go from last or next to last in a conference to conference champion in one short off season thanks to the transfer portal.
Whether the Sun Belt underperformed as a conference this season is a matter of perspective. As a conference the Sun Belt had some great wins vs P5 teams (plus ND) this season and finished ranked as the second best G5 conference. I wasn't one that believed the Sun Belt was overwhelmingly stronger than CUSA when Marshall joined the Sun Belt. I believe that G5 conferences are very close in strength overall with some strong teams and some weak teams in every conference.
Everyone should be able to agree that Marshall is in a much better position as a member of the Sun Belt Conference that what it would be in if it had stayed in CUSA. If Marshall would have opted to stay in CUSA then So Miss, ODU, and another CUSA school (WKU, MTSU, FIU, or a combination of schools) would have left CUSA for the Sun Belt. If Marshall would have opted to stay in CUSA it would have been a death sentence for all Marshall sports. Given the future line up of member schools, CUSA is on life support.