Brompton is a success because it solves an actual problem that people have: last mile transport in a way that actually works, does not require batteries, is reliable, reasonably quick (I have one and once you're used to it you can make pretty good speed on them) and it folds up to the smallest package on the market that is still practical. You can take it with you on public transport here in NL and it doesn't classify as a bike but as a carry on which means no extra arrangements, just walk onto the train and unfold at your destination. It takes 30 seconds and even though they are a bit heavy they are well designed.