>>579293
Only slightly kiked reasons aka counting pennies.
FWS has marginally higher production cost material-wise, is a considerably more complex design for the wing construction and offers somewhat marginal advantages and I think slightly increased drag (for which it greatly compensates with significantly higher lift giving quite a significant advantage in lift/drag ratio).
Also see: >>579276
>the F-5 was built around the concept of a kind of reverse-monocoque. A solid core (engine, fuel, cockpit) provides structure, and then all of the other parts are tacked onto that, and the skin is just an afterthought *makes it easy to upgrade with composites**. This means everything that needs to be maintained is skin deep, just crack open an access hatch and do your job.
I believe is the main reason F-5 won over F-16 as the candidate for FSW modification because, and it's my assumption, there should be some more radical modifications on the airframe, than just replacing the wing, to make it aerodynamically compatible with the FSW, but that's just a personal hunch resulting by observing the existing prototypes compared to their baseline planes and by the airflow given for the configuration (pic related).
X-29 is barely recognizable as a modified F-5.
The Su-47, though a whole new airframe and based on the S-32 paper-concept, is pretty obviously developed from the Su-27 family and dare say more specifically the Su-37, therefore its original designation as S-37.
In both planes we see two same trends for modifications compared to their baseline models:
a) Elongated fuselage, presumably for greater wing attachment area given the higher lift.
b) Much reduced and much more rigid-looking tailplanes.
The Berkut has rather modest-sized tailplanes compared to the Su-27's yuge ones, especially if you consider ruskies' obsession for more and more aerodynamic surface; and the X-29 has, instead of conventional tailplanes, movable butt-LERXs in contrast to the F-5/F-20 that have almost the opposite configuration with nearly cross-shaped tailplanes. My assumption is that it has to do something with greater and denser or fuckier airflow as demonstrated in pic related.