I don't think it's a good comparison to take a back who had an abnormal monster year YPC-wise and use that as evidence he is slipping.
Which of these numbers is not like the other? 5.6, 4.8, 4.4, 4.6, 4.7, 6.0, 4.5.
Anyone who was expecting Peterson to follow up his 6.0 year with another isn't grounded in reality. He is a consistent 4.5-plus back (other than the 4.4 year) and with 5.6 and 6.0 years being anomalies. To use the fact that he returned to normal as evidence that he's slipping is a reach.
Now, losing playing time to injuries is a different story. Fortunately, he's started at least 14 games in 5 of 7 seasons (not counting last year).
But this is the point isn't it? Only Peterson's 2000+ yard season was better than what Murray ran for last year. So all this talk about how Peterson is better than Murray doesn't make any difference, because 'regression to the mean' would suggest that we aren't having another 1800 yard runner next year no matter who is back there.
When people say that they want a dominating running game, how much more dominating can it be than 1800 yards? This tells me that a dominating running game isn't enough to push us over the top. It did help Romo become the top rated passer and Dez catch the most TDs, but it didn't help us win the super bowl because our defense didn't have enough bullets to stop anyone.
So if we have unlimited resources we probably would have just signed Murray, which is what Jones said he would have done btw. But say we had unlimited resources and still didn't sign Murray, then I could be convinced that going after Peterson would be a good option for 2 years.
However, in a world where going after Peterson means that we will have less resources to devote to defense, I can't be convinced that it is a good idea. Even a record breaking year at 31 years isn't going to be that much better that what we had last year. Give me a pass rush, which was the biggest weakness on the team.
Regardless of what people think the Cowboys are going to do, their actions corroborate this line of thinking. They didn't sign Murray to a big time deal and they signed Hardy despite the off-the-field issues. With a one year deal, Hardy is just a bridge to the next guy. Hopefully Lawrence continues to develop and we can draft some more DL this year and next.
That's the plan whether people like it or not.