I wouldn't write off Simpson that quickly. I think UM is going to end up with a 25/18 minutes split between Simmons & Simpson next season (projecting around 3min/gm of both on the floor together). And it wouldn't stun me if it is Simmons getting the high end of the split early in the season and Simpson earning the high end of the split come conference play?
I think Simmons is a very nice get for UM, but I think his upside is above-average B1G point guard (but not All-Conference team contender). I also think we're going to see an improved and more confident Xavier Simpson next season.
As for DeJulius in '18-19, most freshman PGs need a season to really get acclaimated to Beilein's system; I'd project FR year DeJulius as a quality backup PG to Simpson but with more ability to play the two together (because of DeJulius's shooting ability).