Dear "voice for reason,"
You have expressed concern about the Broncos' lack of experienced depth at RB, and you are not alone in that concern. This post may not alleviate that concern, but I hope it will at least clarify what constitutes average as opposed to extraordinary rushing statistics in terms of average rushing yards per game.
Since 1990, a time span encompassing 24 NFL regular seasons, there has only been one NFL season in which more than 3 teams averaged more than 150 rushing yards per game. That was the 2012 season, and the 4 teams who surpassed the 150 yard mark were the Skins, Vikes, Seahawks, and 49ers. 40 teams over the entire 24 year span surpassed the 150 yard mark for an average of 1.67 teams per season.
During that same time period, there has been only one team that averaged more than 175 rushing yards per game. That team was the 2006 Falcons with 183.7 rushing yards per game (the only season in which Michael Vick rushed for over 1000 yards). Two other teams averaged more than 170 rushing yards per game, the 2009 Jets and the 2001 Steelers, so less than 1% of all the NFL teams over the last 24 years have averaged more than 170 rushing yards per game.
The lowest league average number of rushing yards per game, 104.28, occurred in 1994. The second lowest, 106.49, occurred in 1999. The highest league average number of rushing yards per game, 117.85, occurred in 2003. The second highest, 117.31, occurred in 2006.
During both seasons to date with PFM at QB, the Broncos have been well within the average range for average number of rushing yards per game. In 2012 they ranked 16th with 114.5. The league average was 115.9. In 2013 they ranked 15th with 117.1. The league average was 112.9. I see no reason at this time to worry that they will be below average in 2014 with their young stable of talented, albeit inexperienced RBs, especially since my worrying won't change the situation. Instead I prefer to hope that young fresh legs will lead to improvement.
Unfortunately I haven't figured out yet how to post a link within a fanpost, so I will post a link to my data source in the comments. It's a pretty nifty sortable table for certain rushing statistics.