As it stands today, Copp's is tied for 44th highest cap hit among veteran centers. This doesn't account for the time of the signing, so it doesn't consider the economics of the league at the time nor the signing status of a player. There are players who signed with term that have discounted cap hits because their team was buying RFA years at a discount. This places him firmly in the 2nd line center bracket. His output might place him as a 2nd to 3rd line tweener. Under no circumstances is his "best case" a third line center. That's more in line with the floor of his game.
He was not a bad signing, his contract is within reason. You paid open market prices for a UFA center when, at the time, you had Larkin and nobody else behind him playing above a 4th line level. You couldn't afford to not bring in help, and he continues to provide help that you might just elect to ignore because it isn't in a box score every night.
I don't necessarily disagree with what you wrote in terms of it being a fair market price for a UFA but I do believe it was a bad signing for years and amount for this team. You have to overpay in the UFA world to land guys. Its why it is almost impossible to build a team via UFA. Stevie gambled in the same way he did with Compher. Take a chance on a solid defensive guy that will hopefully have an upward tick in offensive after a career year. He has returned more to his norm though, and that is a bit of a problem.
He may have the 44th highest cap hit in the league for centers but he is the 87thish in terms of point production for centers. The year before 72nd (148th all skaters), Even in his best season with Panarin (59th for centers, 100th for skaters). He statistically produces at a 3rd line level.
On teams that have a true number 1 but not franchise elite center ala Larkin, I would argue that you need you number 2 center to produce at least in the higher end of #2s. Copp is not someone that can produce at that level consistently. Even in his best season he was still close to that mid to lower end of that 2nd C group. If he played say Draper level defense, I think you could give those lower offensive numbers a pass but he doesn't. He's good defensively, but he is not elite.
Now when you look at our cap situation go forward we already have 57.4 mil wrapped up. Conservatively to resign Seider (8mil), Ray (5.5 mil), Berggy (1 mil), Ghost (4.5 Mil), Kane (4.5 mil), Veleno (2mil), Ras (2 mil), Fischer (1.26or equiv), and bring up Ed (.897) next season you are looking at probably close to 29.6 mil that puts us at 87 mil. If we want to bring back Sprong you are looking at probably 89.5. That would put us over the cap or just under it with cary over if we were to do some funky Kane 35+ magic.
So if the Wings want to bring in good top 4 or top 6 players they have to move out some of these overpaid (for their role) players to make room.
I should note that I don't think Copp is a bad player. He is a very solid 3rd line, 1st pk guy. If he were the Wings only overpriced guy, it would not be a big deal because you can usually get some savings on other young or rookie contracts.