I can't answer for B.rock but I do agree with him. I've always been puzzled about this. I can't tell you how I would do it differently but I definitely would do it differently. Just seems like a more square box with straighter walls would make more sense, but, Ford already did that (Transit), as did Dodge (Promaster) and Benz (Sprinter). I know the older body on frame argument of course and there are other reasons an E series works for so many.
Also, I'm sure a huge part of SMB's decision to clone an E series had to do with them having the top molds and the rest of the cabinetry and interior bits, measurements, 4wd system, workers with years of experience with this model, etc. etc.
It's just always puzzled me that they took years to bring out a fiberglass copy of an E series.