It doesn't really matter if people keep their vehicle for decades, because people usually sell their old vehicles for use by others when they get a new (to them) vehicle.

What you want to know is the average life of a vehicle (or average age of the fleet), not the average period of ownership.

I'm sure that varies by country; IIRC Germany has punative taxes on registering old cars, so their average age should be a lot less than in the US where registration tax usually is based on value, so goes down as the vehicle ages. I seem to recall average fleet age of about 20 years in the US.