The argument seems to be that people pay more money for products they find more convenient. That's true, but I don't think it's a problem by itself: it's their choice how to spend their money. The problem would arise if the more convenient choice also externalized in some way: if it hurt other people, or the planet, and so on. There is some vague idea that throwing away a cartridge "produces more waste" than throwing…
Cartridge and (quality) safety razors don't last the same amount of time. The additional plastic on a cartridge doesn't seem like much, but when you look at the millions of cartridges disposed in a year, it certainly adds up. Compare the packaging and space of 100 safety razors to 100 cartridges and you'll see an enormous difference. This all stacks up to a bigger impact in packing, shipping, etc.
Not to mention the disposable whole-stick razors that some people use (those are terrible).
You also don't provide any data to back up your claim that the total share of waste from all cartridges is trivial. You can't just hand wave this away so easily.