Well, I think he only has 1 year left on his contract, so they would have to franchise him in order to keep or trade him in 2019. But, they still might get something for him if they franchised him and traded his rights, just like the Patriots got for Jimmy Garapolo. Foles ought to be worth a 2nd round pick in that scenario - IF the team that traded for him could re-negotiate a long term contract with him, the way the 49ers did with Garapolo.
So, despite all the speculation that the Eagles would maximize Foles' trade value by moving him this off-season, it's most unlikely they will do that!
They cannot be certain until sometime in September whether Wentz is really going to be fully recovered from his knee injury. He might even miss some games or at least be less than 100% to start the season. And why should they take that risk, when they could have the insurance of knowing they have the SB MVP to start and get the team off to a fast start as defending champions, if Wentz can't go.
And it has to be tempting to have the same safety net in place in case Wentz get hurt or re-injures his knee during the season. It worked well enough they won the SB, why change anything?