The Tavares deal buys his age 28, 29, 30, 31, 32, 33, and 34 seasons.
Backstrom's contract buys his age 33, 34, 35, 36, 37 seasons.
Backstrom's contract buys 3 full seasons of him at an older age then Tavares deal does, and the Tavares deal buys a lot more of his most productive years, so the comparison isn't really fair, Tavares contract is a far better value.
This contract isn't likely to be a good value, but you can defend the signing by acknowledging that by keeping Backstrom you keep the most successful core in this organizations history together, you give them the best chance at winning a 2nd title, as well as adding division titles, playoff rounds, and personal career milestones. Having Ovi and Backstrom finish their careers as Capitals, and being part of some very big and important moments can be as valuable to a franchise's history and business, as total on ice value. I don't mind that a GM was willing to compromise a little and likely acknowledge that Backstrom was underpaid for a lot of years while not completely handcuffing the team 8 years.
However I don't think you can make a good argument by comparing this contract against contracts signed by younger players. Backstrom is better than Duchene, but that contract is bad, so the comparison doesn't make Backstrom's contract look good. Backstrom isn't as productive today as Tavares so that comparison really doesn't help.
Signing Backstrom is good, and likely worth it, he is likely not going to live up to the cost for the duration of the contract. Both things can be true at once.
There are also intangibles. Backstrom is a winner. It's hard to quantify winning, but its gotta add substantial value.