A car won’t be driving if it only has 20 miles. That’d be a battery failure that prevents the car from operating.
200 becomes 190, 180, 170 … 140.
At least on a Tesla below 140 would be covered under the 8 year warranty as that’s more degradation than should happen (unless you’re over mileage)
It’ll keep going down from there, but there’s a point where the battery just won’t function properly with the car anymore due to voltages or something to do with the level of battery degradation.
So hypothetically when it’s hit 100, it won’t be able to accelerate properly anymore making it no longer viable as a car battery.
At that point you could maybe go into a secondary life as stationary storage which has less demands, or recycle it.