To me it's rediculous how many different cartridges there are, each almost indistinguishable from the ones next to it. I think that 9 cartridges should be enough. A small, medium and large caliber, each with a small, medium and large case behind it.
What is small? .22? .380? 9mm? Does 9mm fall as Medium, or is it still small? What about .40? Are your small/medium/large going to be .22/9mm/.45?
Most of the rounds that are out there in general production have filled some sort of niche. .45ACP for the big boom pistols, 9mm for the high cap crowd, the beloved .40S&W sort of fills in the gap, .357 sig fills in the gap for people who like hotter rounds, .380 and .32 are for the really small and microscopic defense guns, .22 is ubiquitous...rounds like .45LC are anachronistic, but retain value for other shooters. .45autorim would, I'd imagine, let you load for your revolver exactly as if you were loading for your semi, just with a brass swap.
In your 9 rounds, would you sacrifice one of these "useful" rounds for something that people love for fun, like the .500S&W?
And besides, even if there are dozens and dozens of cartridge types out there - why do you care? Dick's and Wally World only stock a handful of the most common ones. Most guns shops only use a tiny bit of space for the uncommon rounds. People that shoot a lot of the non-mainstream rounds either look high and low for them, or reload. They shoot them because they like to change it up, because a historical piece they have uses them, or because it fills some sort of personal niche. I doubt you'll start seeing 9mm makarov replacing all of the 9x19mm on the shelves at DSG.
I see your point, that there are a ton of types of rounds out there and if everything used 1 of the 9 it would be easy to stock it all..but those that choose the oddball rounds accept the difficulty in finding ammo. I respectfully disagree with your position
To keep this mildly on topic, the diversity in ammunition does cause some confusion (like the .38S&W up for karma in the member section), but a glance at the box can mostly eliminate that. It doesn't help that manufacturers don't always call the same round by the same name (Do I have a 9mm parabellum, a 9mm luger, or a 9x19mm gun? Is this a .380ACP or a 9mm Browning? .45ACP or .45Auto?)