$31,000 savings over 15 years comes to about $2,000 a year. Not exactly what most people would consider adequate compensation for driving an old, crappy car over much of that time.
It might make sense if you don't drive around much, make a low salary or are on a fixed income. But most people would rather upgrade their car every 4-5 years and don't mind paying a little more to do so.
For myself, I drive about 40,000 miles a year for my work. I suppose I could theoretically put 600,000 miles on my car as I maintain it well. But the prospect doesn't appeal to me.
If one takes a modicum of care in maintaining vehicles, they may be old, but they will NOT be "crappy". My Integra (1992) has 134K miles on it, and the interior is as good as the day it was new. The exterior paint faded a bit, and I had that redone, but otherwise it's as good appearance-wise as the new Toyota Corolla S that I just bought for my wife.