I've known folks who limp along old cars for years, dealing with potentially dangerous issues with the transmission or suspension. And don't get me started on people who don't keep tires with sufficient tread.
But all these terrible cars are cheap to continue driving in a just-barely-works state. Folks are able to get to their jobs working daycare or fast food in a beat-up 15 year-old car because many places in the US don't require safety inspections, or if they do, only when the car is transferred to a new owner.
If individual car ownership ends, and all the cars are instead owned by large drive-share corporations, that's obviously going to be heavily regulated, cars will all be maintained on a best-practice schedule, etc. Similar to how rental car companies or Uber/Lyft today only use newer, reasonably-maintained vehicles.
If that becomes the reality for all cars in America, we might see a market sort of like Japan. They have a rigorous periodic safety inspection called Shaken, so cars in Japan mostly tend to be newer. Once cars get older and require more work, they're often seen as fairly worthless in Japan and exported to poorer countries with more lax vehicle requirements.
I personally like the Japanese system - it seems to make sense that cars on the road should meet a rigorous level of roadworthiness. But that's so different from the current reality in the US that I'm not sure society would go along with the higher costs that would come with these well-maintained shared vehicles.