We are talking a lot of BF and the high costs of BF bikes.
But, even if most folding bikes are low cost bikes, BF (and Brompton) is not the only manufacturer making high end/expensive folding bikes.
If you look at the Pacific Cycle Reach its also not cheap. Airnimal folding bikes are also relatively expensive. Same for Tern BYB.
The Tyrell folding bikes are even more expensive.
The reason why most folding bikes are low cost is because most folding bike users buy a folding bike for seldom use and short distances, for many of them its a second (or third, fourth...) bike for occasional use.
There is only a small number of folding bike users that use a lot their bike and are looking for high quality, high reliability and high performances.
The first category is people commuting who use their folding bike daily, these are looking for a very reliable bike and for many of them a small and very quick folding bike. The Brompton is their first choice. If they want higher performances, much higher comfort while keeping a small fold, the Birdy is their choice.
Then the second category (which is even smaller than the commuters) are looking for a high performance folding bike easy to carry and as efficient as a not folding bike, these are the customers of BF Pocket Rocket, Tyrell FX/FSX, Airnimal, Pacific Cycle Reach... and probably yje Swift.