This isn't true in general. I know my back-inflate BCD is relatively modular, not quite to the point of BP/W since the "Harness" and "backplate" equivalents are stuck together, however the wing, tank-bands, most of the straps and everything are modular. For now I like all the "bells and whistles" (integrated weight system, pockets, etc., but I think if I ever get into more technical stuff, I can probably use the same wing I have now and just buy a backplate system. It can also take doubles if I go that route at some point.
Zeagle has somewhat blurred the line between the two since their high end BCDs have a lot of BP/W features, although for a minimalist there is still too many extras I'm sure.