Interesting thought, but does it change anything? Each vehicle is still driven the same # of total miles and the same number of annual total miles are driven, so I'm not sure if the resulting carbon output is any different.
From an environmental standpoint, the benefit would be on vehicles where parts get more difficult to find or the owner deems the vehicle to be lacking sufficient safety features (and possibly both, a couple hundred dollar repair prompting the owner to retire the car).
I don't have any numbers to estimate the impact, it's my shower thought for how to maximize asset utilization and minimize vehicle deaths.
What does any of that have to do with the environmental standpoint?
The most extreme example would be a car with a catastrophic failure that will happen at 200k miles, but the owner only drives 1k miles per year. Parts will become an issue far before the useful life of the vehicle is up. The resale value would be basically 0 after 50 years because everything is so out of date, from safety features to creature comforts.