While it's hard to argue that Dak isn't at least partially responsible for the team's mediocre play this year, there are a few things that I find interesting.
1) Dak's QBR of 90.3 puts him in the same general category as Andy Dalton, Tom Brady, Jimmy G, and Matt Stafford. It's not a good number, regardless, but that's some decent company
2) Dak is second to only Cam Newton in terms of rushing yds by a QB and he's tied with Cam for total TDs this year. Neither of those stats is factored into a QBR rating
3) Even with the addition of Cooper, it is virtually impossible to argue that there's a worse receiver/O-line combo in the NFL aside from possibly Buffalo
Again, I won't sit here and say that Dak's not at all at fault because he's made some bad decisions this year, but it's also a whole lot easier to look good when you have weapons all over the field like KC, LA, and even Philly. If Dallas ends up not extending him, I feel safe in saying that there are probably a dozen teams that would love to give him a shot.