I know injuries happen to all programs but take last year for example.

You lose ALL experience. Not one player above sophomore after January or so. Kyle Davis plays less than 15 games (I'm estimating), and LJ Rose misses the last run of games. Having leadership on the team for a college basketball team is so important.

Missions are hard too. I know KC and THaws came back pretty strong (and those are also in conversation of top players @ BYU in last 25 years...). The average returning RM will take a year or two to return. I thought the mission age going to 18 would be a help for byu basketball to give 4(ish) straight years on the team, and maybe there hasn't been enough time to see true results, but it hasn't been that great so far.