I don't like the idea of giving that type of deal to a depreciating asset, but you've made a great point about the discount rate, and the increasing cost per WAR over the length of the contract.
I'm also reading more about Seattle's RSN, and their limited payroll... I guess I can see the rationale.
I still think Seattle will wind-up regretting the deal overall though.
The obvious catch is if Cano is out of baseball after Year 7, it won't matter to the Mariners at that point in time that they got excess value in years 1-4. They'll be spending $24M on nothing in years 8 thru 10, regardless of whatever that money could buy at that point.
His performance in those years has tons of value, but you can't bank it and apply it forward.