As for Fade Rathnik's disertation. One point to keep in mind, that the VF-19 came in 4 variants. The VF-19A, VF-19F, VF-19S and the VF-19P (excluding the Kai as it isn't a production variant). The F & S were both space optimized, hence the fixed, shorter wings.
As for the VF-171, it is entirely possible that changes in political climate could have necessitated a need for a cheap, quickly produced fighter to replace an either aging or possibly reduced number of 19s in service. The VF-25 definitely isn't using a bleeding edge spaceframe design as in previous models, but more a utilitarian design augmented with bolt on, mission specific combat packs. It's advances are more internal and mechanical, rather than aesthetic.