Where a great OL makes the greatest difference is in 3rd/4th and short. Plays that typically are among the smallest gain YPC-wise.
I think it may be most obvious in those situations, but for many of the lines we're talking about, defenses routinely put 8 or 9 in the box which makes success, much less dominance much harder to achieve.
I don't think it's unreasonable to say there was some of both (good line play and good running backs) during the 40+ years I've been watching Nebraska football. Some of it was great, some not very good at all. Some of the best backs who ever suited up for Nebraska were made to look even better by line play and some of the worst lines in that timeframe were covered up for by great backs. I don't think it's one or the other.