Crash, once again,

As for keeping him around: I think it makes sense. The Vikings have already spent years building for the future and I'm guessing from their perspective, the future is now. Spielman has his hand-picked head coach and that coach has his hand-picked staff. They all seem to believe they've found their QB. The expectation from the Vikings (and many others) seems to be that both the defense and Bridgewater will continue to improve. It looks to me like the Vikes believe they've already built for the future and that's where Peterson comes in. They obviously feel he has enough good football left in him to balance out their offense and give them a valuable edge they'd lack without him. I don't see much reason to believe otherwise. 30 is just a number. Bodies don't undergo an immediate transformation at that age. Skills don't instantly diminish. Peterson's always been above average in terms of both his physical condition and his performance. He's human so eventually, his game will drop off but there's no way to accurately predict when that will happen. It could happen this year, next year or several years further down the line. Some running backs have had great seasons in their 30s. If Spielman believes he's built a team that can be Super Bowl-bound in the next few years, I think it makes sense for him to keep a potentially game-changing talent like Peterson around.
Personally, I'm not convinced the Vikings are playoff-bound this season but I'm not ruling it out either and I think their chances of reaching the postseason, and winning, are better with Peterson than without him. Ditto for next year and basically until he shows clear signs that his skills are seriously diminishing.
It's nice to have cap room but the point of having it is to spend that money on good players that improve the team. Peterson certainly fits that description.