I hate to correct @elonmusk, but his assertion that it takes 20 years to replace the vehicle fleet is wrong. This thread tries to describe why. 1/

His 20 year replacement timeline is based in the normal rate of car manufacturing/car retirering and total global fleet size. Both of these will be disrupted 2/

Whilst I don't expect total car manufacturing rate to increase much, with the introduction of cheap autonomous taxis networks, the number of cars being retired will increase very fast especially with people living with urban settings where these networks will become ubiquitous 3/

And in the more rural / commuter belt 2 car families will become 1 autonomous car families.
This will drasticaly decrease the total size of the global car fleet as the rate of cars retirering will be at least twice the size of the cars being retired 4/

As time progresses, legislation increasing taxes on polluting cars (ICE) and restrictions on where those cars can travel to will make them increasingly stranded/decreasing value assets on a household and further increase the retirering rate of ICE vehicles 5/