General aviation is a very wasteful business.
Not as much as the automotive industry. We hardly ever see vehicles over 15 years old around here and this is a poor part of the country. Up-country nearer London, you never see vehicles over 10 years old. Vehicles over 10 years get scrapped because a relatively simple fault such as a worn power steering pump costs more than the car is worth, unless you're an enthusiast where money doesn't come in to it.
Most Cessnas and Pipers in the UK market are over 40 years old and are still marketable, but renovation is as you rightly say never worth the cost, so they gradually deteriorate. A decent 172M is worth looking out for in the UK but is rare as hen's teeth. Look for a French-built example as they were better made than the American versions.
I reckon in the UK a '73 model PA28-180 needing new paint and an engine is worth around US$20k or $60K sorted out for a school fleet. Perhaps they should have put some of those scrap PA28s in a shipping container and sent them over here!
TOO