Carlson and Oshie aren't really comparable at all. Oshie is 3 years older, has a more comprehensive injury history, plays a style that lends itself to more injuries, and plays a position that tends to favor younger players. Carlson is still in his prime for a defenseman, has had one major injury that seemed to linger but he seems to be over it, and rarely takes big hits. He's not Mike Green getting wrecked every game, he's smart about his body seemingly. Also he's stepped up his game most postseasons, which may just be a product of good fortune but either way shouldn't be ignored.
I've kind of settled on the idea that Carlson is a very good defenseman but he isn't able to drag bad partners to good on-ice results like a Doughty or Karlsson are able to do. He's been anchored to Orpik and Alzner for most of his career (with a sprinkle of Schultz, Erskine, and other lugs) and he's still been pretty good in those minutes. When he's been away from those guys he's been dynamite, including this season with Djoos. If Djoos continues to impress and they re-sign Carlson they have a very formidable top 4 going into next season with every opportunity to make smart moves to round out their defense.
$7M+ might be a slight overpay but what do you do with that $7M you save? Spend it on building the depth? Cool, now you have a glaring hole at 1/2D and PP1. Saving cap space by not overpaying is good, but you can easily do that by not overpaying the depth players. That means goodbye to Beagle ($1M savings on a 4C), goodbye to Orpik (probably like $1.5M+ in savings in a buyout + replacement cost, possibly more if you can trade him), goodbye perhaps to Burakovsky if he can't get it together (a few million in savings on a 3W), etc. Don't skimp on your #1/#2D unless you are bringing in Erik Karlsson or similar magically somehow.