So your "theory" is that Donald would reject the 747 currently provided as AF1 and downsize to a 757? That's a fantasy at best.
Actually the 757 is now in demand by the airlines .
The production of the 757 was short and Boeing never made an another craft to really replace it .
They did revamp the 737 but it still falls short of the 757 in overall functions.
The 757 can climb to cruising altitude quickly while the 737 has to slowly climb to one height , burn off some fuel then climb up to another height , repeating this until reaching cruising height.
So having a 757 as AF-1 , might encourage Boeing to bring back the 757 .