You say the AD guys would have preferred a 2seat 15 or the14.
Why the 2seat f15, It appears to have been a successful fighter in it's single version?
Because, despite what some single seat guys say, 2 heads are better than one (usually). If you are doing someting simple such as follow the black line to gps bomb drop, benign CAS or 2v2 then single seat is fine. But when things get busy you can always do better with 2 people.
The problem is that having a second person in a manned aircraft will cost some performance (range, basically) and will add significantly to through life cost, so manufacturers don'tlike it because the glossy brochure doesn't look as good (no box for "capacity" plenty for "multi-sensor data fusion" etc).
It is similar for the 1 v 2 engine argument so basically, Single Seat, Single Engine, the only way to fly cheaply (but you'll get away with it most of the time) hence the configuratio of F16/F35 which have always been aimed at the cheap end of the market (
).
If you can get some data on how F16 and F3 have compared in A-A on large package exercises - most (especially non experts) will be surprised.