Boeing is just like any other ACMI (or in this case CMI) customer, Here today, and we'll see about tomorrow. If Boeing pulls the plug in 5 years you won't hear so much as a whimper as has been the case countless times with other customers. These contracts come and go, it's the nature of the business.
Now, suing a customer because they did not renew with you, not exactly a good tactic if you want to rebid it somewhere in the future. And, no sour grapes required (or in this case lawsuit), just move on to the next gig.
Now if Evergreen wants to place an order for 26 (12 firm, 14 options) for 747-8F's I'm sure that might help them in 5 years minus a lawsuit.