A bike that splits in two works well for packing, but would be a nightmare for other folding uses. I can't imagine how you'd navigate turnstiles with a split bike.

An 11-15 pound folding bike would of course be great, but is probably impossible given the requirements and current available materials. Or if you could do it, it would be insanely expensive, to the point where you'd never want to take it anywhere that an ultralight folder would actually be useful (like a subway or bus).

Engineering is about compromises, not daydreams....