[viernullvier]

Those estimates are unrealistic regardless of any potential degrowth scenario. You'll run out of drivers and/or roads way before the 100 year mark.

[funnymony]

I must concede: 1000x increase in driver count is too far fetched.

Though if forever growth is assumed, and goal post is moved to co2 total instead of per km, then some options exist to eat up that 1000x saving: Owning more than one vehicle per person, buying new automobile more frequently, moving to bigger, better and powerfuler cars.