I suspect animating his more realistic faces and anatomy was the bigger issue. Even a simpler version of Reeve probably would have been a huge challenge for the animators, so they probably just decided it was easier to go for something different altogether.
James Tucker said that was the reason on the commentary track. He initially tried out designs based on Franks' art style, but there was just too much detail for it to work in animation at this budget level. He also felt that the script didn't feel particularly Donner-esque, so a Superman modeled on Chris Reeve, such as in Franks' art, wasn't the best choice.
They also said on the commentary that the coda in the credits was to leave open the possibility of a sequel.
Kai "the spy" wrote:
I liked that they finally managed to get the \S/ on the cape.
The S shield was on Superman's cape in Batman: The Dark Knight Returns, Part 2
, but Tucker said he was pleased to be able to take it a step further and make it two-tone in color. One of the others asked him if technical advances made the S shield on the cape possible, but he said that's not the case. It's just that the foreign animators have improved their skills and can now manage it.