Why should the BB height remain constant? If we look at it from the point of view that a rider who needs 650c wheels to start with will automatically be using smaller cranks e.g. max of 165mm, then the BB can be lower (looking at it from a pedal strike point of view), so the rider will sit lower and won't need that extra head tube or spacers. I.e. is the reason for the longer head tube/extra spacers because of the higher BB in the first place - manufacturers having to accommodate the average 170-175 crank length? And obviously, the lower the rider can sit relative to the ground, in absolute terms, then surely the more aero?