Questions About How Long An Indian Penny Will Last In A Slab

I don't believe that's true with modern slabs. If it was, I don't think anyone would want their coins slabbed! Nonetheless, I keep my slabs inside ziplock bags with a dessicant pack (like I do all my coins.

Talk about under statements. I thought everyone knew it was 86.35744 years.
If you really want to preserve your coins, place them in liquid plastic and allow to dry. Like that stuff people use on table tops. I used some to make a pen holder in 1959 and placed 6 cents and 3 dimes in there. To this day they look just like the day they were put there. Some years ago at a flea market I purchased a Morgan dollar in plastic. It says Continental Bank on the bottom. It's an 1881 S and appears to look like new.
Myself, I keep all coins in either 2x2's, plastic rolls, mostly Whitman Albums in Zip Lock bags. Most of my coins, or all of them appear to look the same as they did when I first collected them. Many now have been with me for well over 60 years.
Not sure what they will look like in a few hundred years but if they start to degrade, I'll let you know.