how long will vintage cards physically last?

[steve B]

It really depends on the paper they're printed on.

T206s may be printed on a cardstock with a high rag content. In other words, it may be more flax, cotton and wool that wood pulp. Rag paper tends to be low in acid and very long lasting. Rag paper was common in the mid 1800's and before, and has been used pretty much constantly. (US currency has a high rag content. )
There's some stuff out there that's 500 years+ and the paper is still supple and not yellowed.

On the opposite end of things, Most of the strip card sets from the 1920's were printed on very cheap cardstock made entirely of wood pulp with nothing to even slow the acidity.
I don't expect that much will be left of them in another 100 years without some sort of intervening preservation - Deacidification is possible, but so far I don't know of anyone doing it to cards.

Oddly I'm of the opinion that encapsulation will make the problem worse. A card that's essentially loose can shed some acid to the atmosphere, an encapsulated card will be contained in an increasingly acidic environment.

Proper storage can help, as can totally insane storage. The proper stuff can be gotten close to in most home environments. I think serious archives try for 40F and 40% humidity. A modern air conditioned home is close enough for most things, maybe a bit too hot and a bit too dry, but not terrible.

The crazy stuff is things like serious climate controls and using inert or nearly inert gasses. In some cases that can extend the expected life well beyond what's normal. But that's expensive I only know of a handful of items stored that way. All of them are on public display in DC and should be very familiar to all of out US members. (I'm sure there are a few similar things in other countries.)

Steve B