I hate to correct @elonmusk, but his assertion that it takes 20 years to replace the vehicle fleet is wrong.
Recently at the interview for his axel-springer award, Elon said that the car replacement would take 20 years. You can see the whole interview here:
His 20 year replacement timeline is based in the normal rate of car manufacturing/car destruction and total global fleet size. Both of these will be disrupted.s