I can not see Disney offering extensions for a few reasons:
1. The Old Key West "mess"
2. Disney is going to run out of spots to put new DVC locations, so they are probably going to want to redo as new.
I think they will target 1 or 2 2042 resorts right away as redo resorts, and the others will just go into the Disney room inventory. Without a lot more rooms added by then, Disney can not take all the rooms at OKW, BWV, BCV, VBR out of inventory all at one shot.
I think if you look in the area of 500$ per year remaining you are in the right ball park, but, you have to consider things like inflation. The value of the contract is going to be dictated by perceived savings over the life of the contract.
An interesting aspect to me is how hard will it be to book rooms at certain resorts? The room supply at Poly and WL was basically cut in half. If demand does not drop in a similar fashion, then the rack rate will go up, and the "DVC savings" will increase.
You could argue that the people who would have wanted those Poly rooms still get them, just with DVC, and to a large degree that is true, but also, there will probably be 15-20 million more people in the US by 2030. How many more world wide? It only takes a fraction of a percentage of that population increase to look at rooms at one of those resorts(poly, WL) to really throw the supply vs. demand situation out of whack.