Whether or not suitable battery replacements exist in 10 years is probably a function of demand. If there's a large demand for replacements, the market will provide. It's probably worth buying a popular model if you plan on keeping your EV for 20 years. For example, you should probably stay away from the Fisker Ocean [1], but I bet Tesla Model 3s will be well supported 20 years from now.
My metaquestion is: is it even rational to keep a car for 20 or 30 years? To me, the subject of the article seems penny wise but pound foolish. Certainly at some point since 1985, an upgrade would have been positive expected value for better safety, mileage, and comfort.
Up until the point that parts are no longer available, or so rare that their cost is prohibitive, it's almost certainly cheaper for him to keep the car than buy a new one. This also includes the fact that he does almost all the repairs himself, so it's also a hobby for him. He's also cannibalizing spare parts from several other salvage cars he has acquired.
A new car has so much depreciation in the first couple of years that it's a terrible idea for most people. Buying used cars and either maintaining them or just driving them into the ground and then buying another used car is almost always cheaper.