Indeed Brompton are expensive and indeed parts are low end and indeed Brompton is looking to make as much money as possible by increasing margins and pushing the bike price as high as possible.
But the Brompton is a UK made bike with its complete frame made in UK with UK labor costs and not in Asia as almost all other bikes. There are very few bikes that are really manufactured in UK or Europe and not only assembled in UK or Europe.
The reason why the Brompton sales didn't increase as expected is not the concurrence of other folding bike, its because folding bikes aren't as popular as expected (the sales of other folding bike manufacturers also didn't increase significantly) due to the huge success and growth of eBikes.
Actually, the biggest part of the growth of the bike market before covid was coming from the eBike growth.
As said, since covid, the sales of Brompton as the sales of almost all other bike manufacturers increased significantly (
Brompton sales increased by about 10000 bikes in 2020 despite a slow down in production during the first lockdown due to missing parts supply).