I was wondering why these kind of blades have been discontinued. Is it because they were unsafe? If so, the kitchen knife is unsafe too. I guess this was because it gave too much freedom to the user. Think about it, today if you have a mach 3 razor, you need the cartridge for that, if you have any other razor, you need cartridges for that. With these kind of blades, you can use any blades in any razor. Good for customers but not good for manufacturers as it won't guarantee repeat purchases.
Going a few decades further back, you can see how it started. First they made you give up your cut-throat razor for a safety razor, they got you to exchange that for a cartridge based razor.