Tesla is already including HW3 in all of their inventory vehicles. Elon's goal is to have a huge fleet of self-driving vehicles, he's going to want his cars that CAN have FSD hardware to get FSD Hardware. For those remaining vehicles that are out there without HW3, once FSD rolls out to a level of a subscription level it's a revenue generator that first quarter. If people already have the HW3 installed, then they can just sign-up and FSD... No need to make an appointment, flood the service centers etc. If it's subscription based, it's a software toggle OTA. Just like many other pieces of technology, all the hardware is built in, you just have to buy the license to activate it.