Finally, sombody said Contender!
Yep, two widely different schools of thought. The KISS and Diversity. Neither one is absolutely the best under all concievable situations. Let's see where each has short comings.
KISS, neat, simple, elegant, simple, lowest cost, and did I mention simple?
What could possibly go wrong with having a single common caliber (or maybe two, one for auto, one for revolver)? Ok, 3 if you count .22LR.
Not much can go wrong, but something to consider, your ammo supply consists of what you have on hand, or if bugging out, what you can carry. balanced against percieved need. And, what you can scrounge/barter/buy.
(and components are the same as ammo for this discussion)
OK, you got plenty at home, you're golden. At least till you run short. Resupply? ok, if it is around. Lots of ammo for the GI, police, and popular calibers, right? But remember, this is the ammo that lots of other people are going to be looking for also! Any of you 9mm shooters that during the recent shortage had to curtail, because the stores never had any, or didn't have much? Somebody else bought what they did have, before you could? Could find .40 on the shelves, but not 9mm? Think on it.
Scrounge ammo from local area or while on the move in shtf situation? OK, you get what you find. Probably you find something you can use, but maybe somebody else got there first. And maybe they took what you needed, and left what was of no use to them (or you)?
Find a deserted home/store and find ammo, even after others hav been there? no 9mm, but maybe 3 boxes of .32acp and one of .25? Point for diversity, maybe. You see the possibilities?
No, it isn't practical to have something in every possible caliber (although it would be fun), but consider the T/C Contender. Only a single shot, true, but besides being a great gun for sport, its still a gun. And having spare barrels in differnt calibers, (cheaper and more portable than complete guns) gives you a good shot (pardon the pun) at having something that will shoot, that could use a wide variety of what you might come across.
For sure, keep a fighting handgun, and basic load of ammo for it, for emergencies, but think about a Contender, for use in Mad Max times. And for use getting food.
The other side of the coin, Diversity, has advantages as well. The down side is the cost (if you are just now trying to put something together), and the lack of portability of the whole bunch (guns + ammo) A plus is having something to use most anything you might come across, and another possible advantage is that you will have trading stock (assuming you have enough to trade).
While it is unlikely that we will ever have a situation where the collapse of our ammo supply system is total, or permanent, it is possible that getting resupply might be delayed, long term.
Imagine for a moment that the ammo makers go belly up, (or more likely are only kept working for govt supply). Now, if you have something in the govt calibers (and are allowed to get ammo), sure, you're set. If you have nothing in govt calibers, you're facing hardship. But if you have guns besides those in govt calibers, you can use them with whatever you can get until that supply runs dry, and still have govt caliber gun(s) and ammo left when you need them.
I've got some really oddball calibers. Not going to get/find resupply in a bad situation (hell, some of them I'm having a hard time getting ammo for today!), but nobody else is going to be interested in my ammo! And I have plenty in the govt/popular calibers too. And then, there's my T/C, with currently, 11 barrels, ranging from .22LR through .45-70. I even have one in 9mm!
I figure, in the bad and scary time, I'll be using those oddball ones first, until I run out of that ammo. Then I'll be back to the "standards", with plenty of ammo for them, too.
You might also consider a flintlock. Its the ultimate long term no manufactured ammo gun. With a mold, and the time, (and assuming you have the knowledge), you can make bullets, and powder, and flints can be found, springs can be made by blacksmiths, etc. ITs the one kind of gun that would still be able to shoot when the rest of the Mad Max world has used up all the factory made ammo (including percussion caps). Now that's real long term planning!