I get that 3-4 years is more reliable in terms of betting rather than 1 1/4 years. I disagree that Peterson will be able to perform on the same level he has for the next 3-4 years though. In my mind, health and a player's 'legs' are critical. ADP's run style is aggressive as all hell. I can't see him staying healthy, running as hard as he has for another 3-4 years.. it just defies logic. A RB's 'prime years' are shorter than ever. That's why I like Foster so much. His situation is likely to stay the same, he is an extremely talented back (I play in a PPR league), and he is entering his 2nd full season, with the experience of finding/hitting the holes hard but not yet at a point where physical health is a concern. Zone blocking gives him bigger running lanes so he doesn't have to go all John Rambo like Peterson does every week. Its going to take a toll on ADP... and its likely to happen sooner than it is with Foster.
Peterson has played in 52 straight games, a few through injury. Projecting him to hit a wall before 30 is not wise. Projecting beyond 3-4 years in a dynasty format, in general, is also not wise.If you think Foster is the safer bet, for other reasons: Great. If you think he is because of injury: you need to do some "Arian Foster + Tennessee" Google searches and read about his history.
I think you need to differentiate between injury and wear and tear. Projecting Peterson to hit a wall before 30 seems pretty wise to me. For the reasons you state - 52 straight games, some through injury. That's a lot of miles for a RB. The question is, if the RB can avoid serious injury in the first place is twofold:
1) At what age will the inevitable decline hit? For many, its 28-29, some even a year earlier. The more a back is utilized while young, the more likely the decline will happen sooner, especially for a big, physical back (like A Peterson).
2) Will the RB experience a decline, hit the wall, or fall off a cliff. We have seen them all, and not like I can tell you which is likely to occur, although I'd wager a guess that a RB who needs their speed is more likely to hit a wall or worse if they wear and tear to the point of losing a step, and their key advantage.
You do bring up a good point in that you can't project much beyond 3 years in FF, and even that can be a stretch. However, if you really are looking long term, Peterson has more miles on him than Foster for example. But imo, should Peterson stay healthy I think we are a good two years from worrying about a decline within a 3 year window.