I think the movement of Running backs to now occupy the top in the last 3 decades really speaks alot. Payton, Sanders, Smith, Tomlinson. Not one punk, thug or headcase in the bunch, all standup guys who truly made(make) this league how great it is.
And not only there durablity, but their likability I think is what made them prolong and get to the point they got to or are going. Emmitt is nothing without the love his oline gave him. Those guys went out there and went to war for him because he gave them just as much love in return. And than we all saw what happen when Hambrick stepped in. Even from the simple transition from backup to starter, something just got lost in the translation. He was a different back because the oline saw what a conceded punk he was and made quick work of showing what they thought of him.
For the other 3 mentioned, can't tell me the same oline/rb lovefest wasn't going on to at least some extent. Even if a couple of those lines did suck something fierce, I bet because of whom they were blocking for that they gave just a little bit more effort.
Just too bad WRs can't be held to the same scrutinizing and reliance on another facet. I bet we'd see more Marvin Harrisons and Jerry Rices if that were the case. Instead they're basically out there freestyle solo shows with noone to answer to. Not even answering to the QBs that throw them the ball, because the reality of it is a WR is a QBs bread 'n butter unless your name is Brett Favre and you churn out WRs like butter.