I don't doubt his value, skills or leadership. What I doubt is his longevity. If I had to bet, I'd bet on cutting him a year too early than a year too late. [Heard of this before too, no?]
Yes. But cutting Mangold means we're rolling into 2017 with three new starters on the offensive line and no veteran making the line calls. Do we really need to free up an additional $10 million when the aforementioned cuts are already going to give us $40-$50 million in cap space? It's not a very prolific free agency class. Unless we're doing so to make a big play for Kirk Cousins. Personally I'd roll with one more year of Mangold so we can settle the rest of the offensive line. Resign Johnson to a multi-year deal so he's ready to take over in 2018.
Note that the Cowboys drafted the RB ahead of the QB. Also, you're right that Petersen by himself - or even OJ Simpson - couldn't take a team to the SB, but coupled with an above average QB a stud RB can make the difference not only for a young QB, but also for the OL since they aren't going to be asked to sit back and get their heads beat in by the pass rush almost every play. So if Fournette is available they should grab him IMO. I agree that they need to get/use TEs to catch the ball. Coupled with a great RB this would make a huge difference for a young QB, and the OL. Of course they have to improve the OL and also find a stud pass rusher, but I think getting a great RB takes priority.
Frankly - as I have stated in the original posting, I'd like to see a few more years of Mangold and Marshall - just not at their current cap figures. [But if that's how they roll it, I have no objection against that either.]