1) you said they replace battery - in tsla battery is custom flat component of chassis. Fuel cells wouldn't replace them at all. b) as range extender they need space - have you seen a Tesla top of the range car? there isnt space to double the battery pack and have a 20lb hydrogen tank. I guess you could sell a car with no trunk for luggage.
Also have to incorporate all the security and safety things for compressed hydrogen
that is why range extenders are for delivery trick not salon cars.