I don't think anyone is going to care about having a smaller turbo 6 over an NA 6. If it's making similar power with better fuel economy then they will buy it.
Now that is not saying do away with the V-8 as an option. There will still be a large segment of buyers who want a V-8 Mustang.
By all acounts Ford is making a smaller and lighter Mustang so I think they will be fine on fuel economy/performance.