It's wrong because there are too many times that players don't last that long, ESPECIALLY at the RB position. Here's a simple exercise to show this. Go pull up any site that shows stats (including this one) from previous years. Go look at the top names at the RB position and see how many are still around and producing at that kind of level. Even the young guys. There are VERY few players that persist more than 3-4 yrs. At the RB position, the only guy is LT at this point. All of the rest of your top guys were either not in the league or non-factors just as shortly as 3-4 yrs ago. At the WR spot, you've got guys like TO, Holt, Harrison (before this year), CJ, and that's about it. At the QB spot, you've got Peyton Manning. Due to this, it's nearly impossible to figure out who the next Holt or Manning or LT is going to be. Every year there are promising stars, some show their potential early, others not exactly, but you don't get a guy because you think he'll be around longer than 3-4 yrs. You get the guy for that 3-4 yr span and if he's around longer, then you hit gold and accept your bonus.
The NFL is too fluid and too dynamic, mostly due to injuries, trades, free agency, coaching changes, and dependence on supporting cast to try and predict further out than 3 yrs. Who would have thought J. Lewis would be a Cleveland Brown or McGahee a Raven just last year? Their situations allowed them to be successful. What if Cleveland doesn't renew Jamal Lewis's contract and he goes to Oakland in a timeshare? Now what's his value?
Ultimately, this is why you try to get TALENT on your dynasty rosters. It's not to say that situations are irrelevant, but if you consistently find good talent, the situation usually works itself out. The goal is to find the LT's and Manning's and TO's and keep them on your roster forever. This is why you want to get a guy like Adrian Peterson on your roster. And you have to do it BEFORE he puts up the kind of #'s like he did this year. Once these guys perform like they do, you will almost never get a chance to get them again.
Far be it from me to question conventional wisdom (I know, *REAL* shocker, right?

), but is it really the case that anything more than 3 years out of an RB is a bonus? I went back and looked at all RBs 25 or younger who finished the 2004 season in the top 30 in fantasy points. Here's the list, along with where they ranked in 2004 and where they ranked last year in parentheses. I also put their ranking in F&L's rankings after the parentheses just to give an idea of how much we expect them to have left in the tank.Tomlinson (3, 1) 2
Domanick Williams nee Davis (5, OoF) n/a
Rudi Johnson (8, 47) 28
Willis McGahee (9, 8) 10
Brian Westbrook (10, 2) 4
Clinton Portis (11, 4) 9
Kevin Jones (21, 28) 25
Chris Brown (24, 45) n/a
Jamal Lewis (25, 6) 21
Larry Johnson (26, 40) 13
Kevan Barlow (27, OoF) n/a
Julius Jones (28, 44) 33
Derrick Blaylock (30, OoF) n/a
That's 13 RBs. Of the 13, three have flamed out entirely. Three more are basically irrelevant (Johnson, Brown, and JJones), although Jones still has some hope of becoming relevant again. The other 7 were either uberstuds still this year (Tomlinson, Westbrook, McGahee, Portis, Lewis), or else they were derailed by injuries (Larry Johnson, Kevin Jones). That's over 50%, and at the top it's even better- of the 6 "young" RBs who cracked the top 11 in 2004, 4 of them still rank in the top 10 (meaning not only have they given us three good years since 2004, but we expect three MORE good years out of them, which would bring their usable career up to 7 years and counting). The guys lower down in the rankings who are still relevant (Jamal Lewis and Larry Johnson) both had a top-10 season under the age of 25, too (so they are examples rather than exceptions). Now, you may point to Williams nee Davis and Rudi Johnson as counterexamples to the "stud RBs have long careers" rule, but remember that 50% of all fantasy draft picks wind up being busts, so a 66%-75% success rate is very, very good still.
For good but not great RBs, then counting on more than 3 years is silly. For the truly elite RBs, the guys who crack the top 10 prior to the age of 25, it's not silly at all to expect more than 3 years of production. For someone like a Maurice Jones-Drew, or an Adrian Peterson, or a Joseph Addai, or a Marion Barber, or a Frank Gore, or a Steven Jackson, then absolutely you can expect them to remain productive until age 28, 29, or possibly beyond.