get a 10mm or larger

There are smaller animals than grizzlies that are large enough that 9mm harcast does not provide adequate expansion. And some of us don't want the larger and heavier frames.

Like I said, if your only standard is self defense using the FBI standards, 9mm is pretty hard to beat, but that's because the penetration is so limited with anything under 12" in gel being considered inadequate, and anything over 18" being considered too much. So any modern loading for any cartridge will sit somewhere around 16". Essentially all you're optimizing for is mag capacity and recoil at that point, because they all have essentially the same expansion as a percentage of original diameter.

But look at the extreme ends like the +P hard casts from Corbon and Buffalo Bore. 9mm really isn't even close to what the other two can do because of the higher weight to frontal area of the bullets.

Sure 10mm can do everything .40 does... Except for fit in standard size frames.

You mentioned earlier that you don't know why someone would want to swap calibers in one gun. Largely, because I can. Also, for what this specific gun is used for, I don't want to buy multiple, I'd rather swap to a 9mm barrel if I just simply want to shoot soda cans with cheap ammo, 357 for shooting small groups (I've found it suited better for target shooting than 9mm and 40 personally, mostly just preference) and generally when I'm out hiking it's set up for .40 (although sometimes I'll keep it 357 for this as well).

My CCW is a G43x if it matters. Like I said if you limit your use case to one specific set of criteria, of course the advantages of different cartridges will seem small. But when you push them to the extremes you'll see why one is better suited to different uses.

The reason those calibers aren't more common is because most people want guns for self defense, relatively light recoil and they want cheap ammo. 9mm does all that fairly well