When did 5.0 YPC over the course of a season become the standard by which RBs are judged?
Emmitt did that for 1 season in his entire career.
Since 1980 only 5 players (minimum of 15 carries/game) have done it for more than 1 season and only 2 players have more than 2 such seasons. Barry Sanders has the most at 5 seasons.
5.0 YPC is ridiculous. If that's the standard for a RB that can be had for a 6th round pick, who in the hell is worth it? Nobody.
I think your qualifier of 15 carries/game is the issue more so than the average. Not sure how you're setting that parameter.
For instance, in 10 seasons Jamaal Charles has only averaged less than 5.0 YPC
once. Granted, some truncated injury years attributed to that, but your 15 carries qualifier negates his 2010 season in which he averaged 6.4YPC on about 14.75 carries/gm.
Off the top of my head LeSean McCoy is another guy that has more than two seasons over 5.0. Tomlinson is probably another guy. Hell I think DeMarco's done it twice.