Why did you think it was unstoppable? Stewart has only ever suited for 16 games in 3 of his 7 previous seasons (2 of which were his first 2 years and 1 in year 4, with a total of 6 starts between those 3 years). Stewart has never started more than 8 games in a season. He was basically a TD vulture in the beginning of his career then just being the perennial injured player on a team that has Cam Newton taking most of the run chances (and TDs).
Miller has been a disappointment if you were expecting his performance of last year, but last year was the only year in his 3 years (prior to 2015) which was like that. He's currently just off the pace to match his 2013 season. Shouldn't be a shock that he isn't matching his best season ever.
CJA had a new OC, a new HC, 4 offensive linemen who new or playing a position they didn't play before, Manning who is rusty and, as he showed last year, has trouble throwing deep early in the season so defenders stack the box, and was against 2 of the top 3 run defenses from 2014 in week 1 and week 3 and the #7 overall defense in the league in week 2. CJA also only won the starting job last year after both players ahead of him got injured and ended up playing really well against a really easy schedule for run defenses.
Gurley is a rookie so thinking he was going to start off great is pretty silly (especially after watching how StL did with the run last year).
Woodhead is #2 on the depth chart so even if he does well he's still a flex consideration at best.
You knew going into the draft that Foster was injured so not sure why you're adding him in the list of disappointments. In fact, he's actually back a lot faster than most people ever thought he would be so that's a huge boost in your favor.
It's just silly when people draft high risk players with no history of production and just go off hype and one year (or half a year in CJA's case) production as the only basis of their draft. If you draft high risk players without any historic proof that they are legit RBs don't be surprised when they fall flat.