July 5th, 2013:
Trent Richardson, Doug Martin, CJ Spiller, Ray Rice
July 5th, 2012:
Darren McFadden, Chris Johnson, MJD
July 5th, 2011:
Rashard Mendenhall, Peyton Hillis
... I can go on and on. Of all the positions, RB is the riskiest elite asset to hold, period. I had a dyno startup last year where TRich and Doug Martin went ahead of AJG, Dez, Charles, McCoy (and later on, after TRich started horribly in Indy, he was traded, straight up, for McCoy).
If I have a young, elite RB, I am trying to turn him into an WR or two, every chance I get.
Just because running backs get hurt, doesn't mean that you don't need them. The McCoy, Peterson, Charles and Lynch owners made the playoffs in just about every league I was in last year. The AJ Green, Julio Jones and Dez Bryant owners didn't.
2) The thing is, while there are some truly elite receivers in the league right now - guys like Harrison, Moss and Owens gave way to guys like Andre Johnson and Reggie Wayne, who gave way to guys like Calvin, Dez and AJ Green - Keenan Allen isn't in that elite tier. He's in the 11-20 range. Here are some guys from that same range over the past few years:
2013: Larry Fitzgerald, Mike Wallace, Percy Harvin, James Jones
2012: Mike Wallace, Percy Harvin, Kenny Britt, Greg Little, Justin Blackmon
2011: Kenny Britt, Dwayne Bowe, Percy Harvin, Jeremy Maclin, Stevie Johnson, Brandon Lloyd, Reggie Wayne
Not all of those guys were TOTAL busts, mind you, but neither were the ones you listed. Ray Rice had four straight years of stud RB production and may have two or three more left in him. Chris Johnson six years of 1000+ yard seasons including his 2000 yarder, and may have two or three more left in him. During that time he's had a year and a half of down production, but otherwise been everything he's supposed to. MJD had six years of top RB production. The jury's still out on Richardson, but Martin still holds his elite trade value. Spiller didn't hit the ground running, so he isn't really a comparable to Lacy, who did. He had a stud season in year three, then played hurt last year in his fourth year in the league.
You also ignored guys like Adrian Peterson and LeSean McCoy. But that's because you were looking for busts. So I didn't list some of the true stud talent that came out of those tiers - guys who were undeniable hits at RB or WR. There are instances of both at both positions. Cherry picking one or the other for the benefit of your argument doesn't help anyone.