When you look at military history, the simpler plan tends to be the more successful plan. Simple plans have fewer points of potential failure than do complex plans.
Yes, it's fun to have a variety of guns and calibers. But the topic isn't what is fun; it's what works best in case of bad times.
When you diversify, you create a minimum of four expenses: the platform, the ammo, the spare parts, and the practice to become proficient. Your platform costs money. Your ammo costs money; without it, your platform is useless. Your spare parts cost money; if your platform needs repair, it is useless without spare parts, and the ammo is useless too. Practice to become proficient costs money and time in terms of ammo, spare parts, range fees, and trigger time not used on another platform. If you don't practice, you may not be sufficiently proficient when the need arises, which would defeat everything you've done.
If instead you apply the costs of the other platform, its ammo, its spare parts, and its trigger time to one platform, you can concentrate those resources more efficiently and effectively. That's why it's a good idea to have more than one of the same platform. There is no additional expense in terms of ammo, spare parts, and trigger time. You are more likely to become more proficient because "practice makes perfect" (or at least gets you closer). It is simpler to maintain and account for. The additional copies of the platform are there to either take over if the primary goes down or act as organ donors, thus increasing your spare parts.
I might also add that although I believe in "Lewis and Clark" mode (foraging off the land; in this case, seeking out ammo during bad times), I would feel more secure making my own preparations than in taking pot luck. You are unlikely to just "come across" a cache of ammo. Chances are, if someone else has a cache of ammo that might work in my diversified collection, that same person has enough to fend off anyone who wants to forage it. Either that person will defend it or take it with him if he leaves, or someone who has killed that person has already taken it and now has it to defend himself.