When we've had successful RB's recently who were picked in Mason's (75) draft range (Charles - 73, Murray - 71, Ridley - 73, Greene - 65) it shouldn't that strange to think Mason could be the starter at some point in the future.
Since Charles in 2008, there have only been two RB's in that range who didn't pan out - Glen Coffee (74), who quit after his rookie year saying God told him to and Ronnie Hillman (67).
This is laughable. Cherry picking recent success in that range means nothing unless you honestly believe a strong correlation exists between that range and NFL success (hint: it doesn't). Guys drafted much higher than those picks ended up busts. It is pure coincidence that those 3rd round guys panned out. Here's a comprehensive list of guys drafted in the third round
or higher since 2010:
2010
Spiller (9) - 1 top 24 season (#7)
Mathews (12) - 2 top 24 seasons (#7 and #12)
Best (30)
McCluster (36)
Gerhart (51)
Tate (58)
Hardesty (59)
2011
Ingram (28)
R.Williams (38)
Vereen (56)
Leshoure (57) - 1 top 24 season (#20)
D.Thomas (62)
Murray (71) - 1 top 24 season (#8)
Ridley (73) - 1 top 24 season (#10)
A.Green (96)
2012
Richardson (3)
Martin (31) - 1 top 24 season (#3)
Wilson (32)
Pead (50)
L.James (61)
Hillman (67)
Pierce (84)
2013
Bernard (37) (#16)
Bell (48) (#15)
Ball (58)
Lacy (61) (#7)
Michael (62)
K.Davis (96)
----------------
Stacy (160) (#18)
TL;DR - Stacy has likely already had more top 24 seasons than Mason will have.
Mason
could be a good RB, but if you are betting on him, you are betting on some long odds. If I had Mason, I'd do what I could to acquire Stacy. If I had Stacy, I'd acquire Mason if the price is right, but there's at least a 75% chance he's nothing more than an injury handcuff at best. Would not be shocked to see Cunningham have a better career than Mason. Just being realistic. Go back farther than 2010 if you want. The odds of a 3rd rounder finding NFL success aren't as good as recent history suggests.