“The double edged razor has become a product that you can only buy on the internet, and which you will only buy if you know that it exists.”
Cartridge heads cost about £1.75 each and work out to around 45 pounds a year. Double edged blades run 8 pence and about 10 pounds a year, with a fraction of the plastic. The cheaper, better product vanished from shop shelves because nobody makes money selling it. That is the razor-and-blades model in its purest form.