I'm personally driving a 2010 Civic which just crossed 100k miles, and a 2009 CR-V with 140k miles. The Civic will likely be sold in the next few years pursuing an upgrade to safety while it would still have a ton of life left in it. It'll probably go to some high school kid who is learning to drive, and its end of life will more likely be due to a crash.
I just put money into the Civic for wheel bearings (likely needed because it sat most of 2020), transmission fluid, and brake fluid, and the CR-V checked out fine at its oil change. In ~75k miles, we have only done front suspension, tires, wheel alignments, oil changes, and a set of brakes on the CR-V. It spent a couple years in a town with terrible roads, which I point to as the primary reason for the suspension breaking.
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 The resale value would be basically 0 after 50 years because everything is so out of date, from safety features to creature comforts.
I know an older couple with two vehicles from 2008 with 36k and 54k miles on them. They are in great shape, always parked in the garage at home, etc. While their natural life might be 150k, it's very likely they will be in a junkyard before then. Part availability becomes an increasing concern.
From an environmental standpoint, old cars were manufactured under less stringent emissions standards. At a certain point, the emissions required to manufacture a car are more than offset by the marginally lower emissions from a new vehicle.