Thanks, but that ultimately meaningless to the subject possibility.
If Bethesda's next project is a long ways out, and with Mantle being an emergent opportunity that didn't exist before lifetime patches completed, adding Mantle support is a means to boost continued and renewed life in their latest flagship product (for non-MMO players), and keep the brand in people's sight on gaming news websites and online-retailers.
Those are the things which matter more than whether their pre-planned support schedule, which completed.