The problem with not owning a Brompton is that you don't get to look at one up close and see its gearing limitations. Spreading the chainstays as wide as 126mm would probably mean a new rear triangle will have to be built. This also means that the B. would not be as compact to fold, thus negating its main attribute. A modern derailleur would mean that the bike, when folded, would not tension the chain properly thus causing the chain potentially to drop off the rings when unfolding.

Sure all these limitations can be remedied (just look at Steve Parry's stuff). But is it worth it, and on whose dime?