His decline hasn't been linear, it's much more subtle. It was obvious this year when Steen and Oshie were off his line. He can't lead a line anymore. For Backes to be successful, it's about how he's used. Put him at RW with linemates that can create some offense. Backes can still finish, create havoc, and be a great possession player.
The problem is if he plays center and/or has linemates that can't create offense for others. Use him the right way, and he should be productive for 3ish years. At a certain point, he'll become an anchor on the cap, but it's just a guess at when that will be. He could potentially still score by being a net-front presence on the PP and in a sheltered role at the end of the contract.