Remember the battery has individual cell’s. I don’t think all the cells will need to be replaced. If they failed during the warrant period one at a time you should never have a major problem.Don't know how much a complete battery replacement will cost. Maybe in the $20k ballpark? The battery on MME isn't a single component -- it can be opened by an EV trained mechanic and the mechanic can replace the individual modules that are contained within the giant pack. That can reduce costs if only part of it has failed.
Also, I'm guessing extended warranties will be available from Ford and third parties, like usual. Stuff like this: https://libertybellauto.com/coverage/