Henry was totally wasted that year. It was Yeldon, and he fumbled more than any other Saban RB. No idea what Saban was thinking there with Henry on the roster. But Cooper was the offense. Take away Cooper and your offense stopped because Yeldon could be shut down.
ETA - just looked up Cooper's stats that year - 124 catches for 1,727 yards and 16 receiving touchdowns. Let that sink in.
I'm not saying that Cooper wasn't absolutely dominant (he was - particularly on a volume basis); I'm simply saying that the rushing attack wasn't as bad as you're implying.
Let's consider some additional data points:
- Yeldon clearly struggled with fumbles early in his career, but he only fumbled twice in 2014 while Henry fumbled once (and they both lost one).
- Henry averaged 0.8 yards per carry more than Yeldon in 2014, but one could argue that difference could likely be chalked up to the fact that an often-fresher Henry was the back used late in games to drive the dagger into already-exhausted and out-matched defenses.
- Don't get me wrong, Amari is incredible (despite his rather pedestrian career in the NFL thus far - here's hoping for a 2018 rebound!), but that doesn't mean he was head-and-shoulders above the rest of the team in terms of production-per-touch. Actually, he simply out-volumed everyone else. While I can't find target statistics for 2014 (I figure he got the lion's share of those as well), he caught 43% of all the passes pulled in by Bama receivers and accounted for 44% of all of Bama's receiving yards - an average of 13.9 yards per catch. The rest of UA's backs and receivers had 2163 yards receiving on 166 catches - an average of 13.0 yards per catch. Thus, the rest of Bama's receiving core was basically as efficient and effective on a per-catch basis as Amari was. [Yes, one can also make the argument that Amari drew tougher coverage; one could counter-argue that that number includes dump-offs to backs that hurt per-catch averages, but this has to end somewhere].
That's a lot of numbers, but it all circles back to my original counter to your original point (and again, I'm not saying that Amari wasn't phenomenal): saying that the running game in 2014 was pedestrian simply isn't accurate. Having two thousand yard rushers forced teams to load the box and helped to open up lanes and space for Blake to air it out (mostly to Amari) on a record-setting level. He certainly wouldn't have been able to do that if teams were dropping six and seven into coverage on most plays.