I think there will only be "grumbles" about running back by committee if there is no production to speak of.
If each RB is averaging 5-6 (or more) yards a pop, I doubt any one will care.
This isn't a "Rozier gets 40 carries a game" offense, either. Too much of its strength comes from forcing a defense to consider all of the possibilities.
As to production, this staff means it when they say every day is competition day. Who plays game 1 may not be who plays game 8. That's where it's huge that Bryant and Washington are now available. There's now competition that forces everyone to get better, and if you're not cutting it (see last year), there are now options. Riley, by his own failure to correct roster issues, was forced to overuse the one back that was productive.