I had the same concern, mostly with the older cardboard that's got a lot of wood fiber.
Some degradation happens to that cardboard, but as with other items that are similar it's not well understood (maybe by the better conservators)
One thing that affects it is exposure to air. So for example, an old book from a period when they were using cheap paper, the page edges will brown before the areas closer to the center. And just for it to be contrary, degrading items that are in a sealed container have the opposite thing happen, where the acid produced by that degradation can't vent, and causes further degradation.
I also wondered about the slabs, thinking if they were sealed they might retain those acids, and end up causing problems fairly quickly.
I asked, and as far as I know, the slabs are not airtight. So those acids can to some degree vent and not be a problem.
Or at least, only as much as the typical storage boxes most of us use for non-slabbed cards. I have some that are well over 25 years old, older than grading and they have not caused any problems yet.
I'm no longer concerned about a regular card in a slab, and maybe a bit concerned about altered cards in slabs, or much older ones on acidic cardstock that are slabbed.
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