It is true that cartridge-razor sellers do try to maximize profits (of course!), but it is by offering razors that will have unique features and benefits that their customers will appreciate.
The fact is that Gillette and others do extensive product research and development, and that includes panels of shaving testers, who evaluate and compare existing products to new ones being considered for release to the public. The panels of test shavers evaluate these products and truly find them (that is, the ones that make it into the stores) superior in enough aspects that the razor marketers feel confident selling these new-and-improved products.
|The Dorco Pace-7 razor:|
the world's first seven-bladed design.
What makes the two-, three-, four-, five, and even seven-bladed cartridge razors superior in some respects is the concept of hysteresis, which is the dependence of a system's output on past and present inputs. In terms of the multi-bladed cartridge razor heads, this hysteresis is a blade in the razor that first arrives at a hair shaft extending the hair outward as it cuts, and subsequent blades engaging the hair shaft before it can retract completely into the skin. This sequential extending and trimming of a hair with a single razor stroke allows the blades to be positioned in the razor head so that they generally have very light contact with the skin surface. Further, pivoting-head designs have evolved so that it is very easy for the user to maintain the razor head in optimal orientation on the skin for the best shave. These same pivoting features make it difficult for the user to exert enough force of cartridge against skin to make the shave unduly hazardous or harsh.
All in all, this means these cartridge razors give a very good shave in a single pass.
That doesn't mean that they are without drawbacks, however. They can be pricey. They can clog easily and be difficult to unclog. They may have a shorter useful life span than one would prefer in order to get the best utility from the investment in the instrument. They are also made of plastic, are disposable by design, are not recyclable, and therefore have a small but negative environmental impact. [UPDATE: They also tend to encourage in-grown hairs.]
They also make getting a close, comfortable shave easy -- no challenge at all. Many men -- most probably -- would see this as a benefit. To others, who enjoy the game of seeing how good of a shave they can get due to their own skill, this virtually-guaranteed, easy, comfortable shave is something of a drawback. It turns the morning shave from a focused, zen-like minor challenge, a daily game, into a quick, mindless, boring daily chore.
Now I, for one, am a confirmed double-edged-razor user. I think it's fun and a small daily challenge to see how good my shave can be (and it's always adequate). I also like the economy of the process as well as the fact that my razors will last virtually forever, and the blades are both long lasting and recyclable.
Is double-edged shaving for everyone? I don't believe so. It clearly does have its place in the market, however -- as do the various cartridge-razor options. As the old saying goes: you pays your money and you takes your choice.