As for Dez - the most value (raw VBD) a player will ever have is as a rookie. Meaning, the best time to have acquired Peyton Manning, Ladanian Tomlinson, or Randy Moss would be before they played a game. You then get all of their career VBD. So, once you identify a player with superstar talent, and you are comfortable projecting production to continue for a long period of time, I think you need to rank that player based on your projections.
Many missed out this past season on guys like McCoy, McFadden, Newton, Dez (myself), Wallace et cetera, because they needed to wait and see. Many treat rankings as a lifetime achievement award - they won't value McCoy over Peterson, because of what Peterson HAS done. They won't value Newton over Rivers or Brady because of what Rivers and Brady HAVE done. I prefer to value players based on what they WILL do from that moment on. Once I think a guy is elite, and am comfortable projecting elite production, I act/rank/trade/draft accordingly. I took a lot of flack for taking LeSean McCoy as the #1 overall player in a dynasty draft, because other guys "have done it for years". I don't think people understood that having done it for years does nothing to your future VBD. Now, those posters would gladly trade the older guys for McCoy - they didn't act on what they saw/projected and it cost them.
What they have done in the past is a very useful and relevant data point (actually many data points, depending on how many seasons they have played) if used in the right way, however. I rank Adrian Peterson ahead of McCoy for a variety of reasons and past performance is among them. Peterson has yet to fail in any season or in any situation (something that can not be said of McCoy, at least to some extent). When I watch both Peterson and McCoy play, my eyes tell me that Adrian Peterson is a more talented player. When I combine that talent with the realization that Peterson has succeeded to a high degree several seasons in a row without any failures, it inherently adds value to him.I don't believe it is fruitful to project any non QB out further than 3 years. Too much can change over that period of time to have any level of exactitude (I feel slightly more comfortable doing it for WR's than RB's, but even then I try not to go too much further out). If you can reasonably expect a player to maintain his current level of play for 3 years, and if that current level of play is equal to another player, then it behooves you to include past performance into your analysis. What the past generally tells me is that I will end up right if I choose the player with the track record (again, assuming it is reasonable to expect 3 more years of the same level of play, which is an important distinction). If I am selecting a more unknown commodity over the one with the proven track record, I may end up being right, but me being right only equals the same level of production I could have received from the player with the track record. However, me being wrong means I left an awful lot of value on the table by passing on the player with the track record to instead select the rotten egg.
This same thing led an awful lot of people to jump aboard many bandwagons in the past, only to get badly burned by them. Rashaan Salaam, Anthony Thomas, Kevin Jones, Julius Jones, etc... were all players that likely would have been selected ahead of proven veterans (who still had a reasonable expectation to maintain their level of play for the next 3 years) using strictly what you are talking about and all would have been severe mistakes. There are countless more examples beyond just those players. For every Chris Johnson, who inexplicably performs under his expectation while still in his prime years, I can point to 10 young players who had a big season and never again played to that level.
Particularly at running back, the landscape changes much too quickly to bother going out further than 3 years. MJD, who was considered as solid as possible until at least his 30 year old season as recently as last year, is suddenly a deteriorating dynasty prospect. Had we projected him out to his 30 year old season 2 years ago, we would have started badly miscalculating his value as soon as this season (with yet 3-4 more years to go until he reaches 30). My point is that, no matter how young a player is, it rarely ends up being fruitful to project any analysis outside of the current 3 year window. This is what leads me to value Peterson over McCoy. I believe Peterson will perform as good (actually better) than McCoy over the next 3 years. If you value McCoy because his current production will last longer than 3 years and you end up being right, I would be fine with that because in my experience, projecting further than 3 years out will lead you to be wrong more often than you are right and will end up costing you overall value.
I also acknowledge that every so often, a prospect comes along that forces you to ignore any past trends or performances and instead value that player over the proven commodities. This is due to these players having such an obscene and obvious talent level that you don't need any track record to feel comfortable projecting them. In recent memory, Adrian Peterson and Calvin Johnson are the 2 that come to mind. Perhaps this year Trent Richardson and Andrew Luck will join them?