Well, they could further downgrade the LoD for AI cars, but they already look pitiful on base Xbox One, the detail was already cut dramatically going from Forza 5 to 6 and then 7, at some point you reach a subpar result. Like I'm not arguing that it can't be done, hell you could probably port Forza 8 to OG Xbox if you really wanted, the question is the quality of the end result and the returns.
Xbox One, PS4 and PC so far have comparable HDD bandwidth, and players on PC with SSD installed are already familiar with being the first to load into a match and waiting for others with lower specs. Again it kills the point of upgrading in the first place, if everyone in a session has an SSD then games could start dramatically quicker. That's the point of nextgen hardware - bringing an experience that wasn't possible before due to hardware constraints. I don't know what the argument is at this point.