My capper was full and I also had a half full tin of #11 caps in the pouch. They had been there 6 years. I didn't expect them to work. But they did. Not a single misfire or slow fire. That impressed me. Those should be about the easiest firearm componant to destroy with poor storage.
That might be evidence that primers will stand up to that kind of storage conditions but based on MSDS comparisons, percussion caps are a different blend of ingredients than primers--so maybe not.
But even if we accepted, for the sake of argument, that primers and percussion caps have similar storage condition tolerances, it's still worth remembering that a complete round of rimfire ammunition consists of more than just priming compound.
I'd be pretty interested to see your test repeated using half a box of .22LR instead of half a tin of percussion caps.