Batteries USED TO BE warrantied for about eight years, don't know about present ones, so it seems that 20 years may be a little optimistic. As there aren't any Teslas out there that are 20 years old, maybe you can be the one who tests this parameter.
The battery warranty is for 70% at 8 years or 100k / 125k miles depending on model. The OP was originally asking about 20 years or 200k miles. while a battery may be at even 50% capacity in that time, that wouldnt make the car not drivable.