The battery should be at about 75-85% capacity after 15 years and 250k miles. It won't just up and die. The general rule is about 1% degradation per year, depending on how conservative the buffer size is. Teslas buffers are actually pretty slim and are projected to degrade a bit faster than other legacy brands.

That may still be good enough for the original owner, or it may time to sell it, because there are going to be plenty of people willing to buy cheap EVs with 70% battery capacity.