I'd do it under the condition that Peterson reworks the contract to a 3 year deal that does not exceed 10 mil annually. Unlikely he goes for that but it has to make sense for the Cowboys financially too.
Peterson would make a huge difference in how the opposing defense can gameplan. He's a superior runner to Murray, and I think highly of DeMarco. Even if you believe Peterson has lost a step at age 30 and after a year layoff, and that's up for debate, he has not lost the rare power great vision and instincts.
As others have stated, when you're a team on the cusp, sometimes you take a risk to put the team over the top. Seattle did it with Graham. You're telling me it's ok to trade a 1st round pick for a TE who MAY not gain 3000 receiving yards over 3 years but it's not ok to trade a 1st for an elite RB who very likely will gain 4200-4500 rushing yards over 3 years???? I believe that Peterson in year 3 as a Cowboy will be a more valuable player than Graham in Seattle at the same time.
Again, money is the sticking point. The contract would most definitely have to be re-worked.