Here's my take (warning long post ahead): One of the reasons JB has been able to keep such a tight rotation every year is he has usually had multiple players playing nearly 35 minutes per game at the guard or wing spots so he has only needed 1 backup to cover the leftovers.
Going back to 2009, every season but two has had multiple guys playing 34 minutes or more, and often there has been a third right below at 33 MPG...Here's every player under JB that has played more than 34 MPG:
2009-2010: Junior Manny Harris (36.1)
2010-2011: Sophomore Darius Morris (34.8) and junior Zack Novak (35.0)
2011-2012: Freshman Trey Burke (36.1) and sophomore Tim Hardaway Jr (34.2)
2012-2013: Sophomore Trey Burke (35.3) and junior Tim Hardaway Jr (34.8)
2013-2014: Sophomore Nik Stauskas (35.6) and sophomore Caris Levert (34.0)
2014-2015: Junior Caris Levert (35.8) and sophomore Zak Irvin (36.3)
2016-2017: Senior Derrick Walton (34.8) and senior Zak irvin (35.4)
Pretty much all these guys either:
A) declared for the draft after the season (or could have)
B) were upperclassmen/good defenders like Novak or Walton and Irvin last year
Really the only outliers here are sophomore Levert (who made an unexpected huge jump in minutes and performance) and sophomore Irvin (who was forced to play major minutes due to Walton & Levert injuries)
Stupidly long post (sorry Dylan)...but I guess my point is, there's a difference between a 35 MPG star that we usually have had multiple of and a 25-30 MPG rotation-lock starter like a Stu Douglass. Just because we have guys returning that have been contributors doesn't mean we have these locks that are basically playing the entire game and can mix and match with only one backup covering for all the starters.
Simmons - 30
MAAR - 30
Matthews - 30
Duncan - 25
Wagner - 25
If X gets the other 10 at backup point guard all to himself, Davis gets the other 15 at backup center, that still leaves 35 minutes behind the other 3 to split. There's no reason to say "it has to be either livers or one of Poole, Watson or Brooks". A 9 or 10 man rotation makes more sense on paper to me with the current roster barring a major leap from anyone.