I guess the business I'm in makes me look at it in a different light.
One of our greatest challenges is dealing with people who want to honor their loved ones last wish to be cremated and scattered. A lot of times they don't want to do it because to them it symbolizes throwing the person away. For that reason we tell families (we use the word "families" instead of "customers") who have lost a loved one that for their own good they should place a memorial somewhere. Dad wanted to be cremated and scattered in the ocean? Great, go for it, but go to a local cemetery, or church, and place a marker there. Or donate a bench to a cause and place the loved ones name on the bench.
We've found over and over that people need something physical to see and to touch that reminds them of their loved one.
Yes, the Shelby was a rotting hulk but the old guy could go out and touch it and that must have brought some comfort to him. Of course we also tell people to not keep the memorial in the house (or the yard in this case) because seeing the object, whatever it is, every day can actually slow down or stop the healing process.
So, was it unhealthy for the father to keep the memorial to his son in the yard? Probably, but can you imagine the pain he would feel if he saw someone else driving that car? Maybe he should have crushed it and kept the VIN tag as a memorial and placed the tag in a cemetery.