I think you make valid points, as I think one and three are most likely the biggest culprits. Though the front fascia is acceptable I don’t think it is particularly good looking. I personally think it is bad enough that it should be redesigned for 2026. The current design does not feel cohesive to me and the leading edge of the hood is too flat.
With the discontinuation of the Camaro and the Charger/Challenger, the Mustang should have seen a sales increase, not a decline. Not every person in that segment is brand loyal, and since you are the only kid on the block now there really isn’t an excuse.