I totally agree with Stoic Joker. When I was in the military, my unit had several brand new cars at our disposal, some were diesel, some were standard fuel.
The rule was that for us to do our job reliably, standard fuel cars needed to be replaced after 100.000 kilometer, the diesels were re-assigned to different duties after 150.000 kilometers. There was only one car (my favorite, a diesel) that was longer than a year with our unit.
Of course, it was the military, so the cars were not treated as "maintenance-friendly" as they could have been. Same attitude people have with lease-cars, I guess. All in all, the diesels were better survivors than the standard fuel cars, with a lot less maintenance costs. But in any case, 100.000 kilometer is really a "magic" number for any car.
Modern cars are not really more efficient with fuel than older cars. They're about the same. New cars are much heavier nowadays, because of all the safety features, noise cancelling padding and electric gizmo's that people take for granted now, but were highly coveted options a few years back.
With some TLC old cars last just as long as new ones. I heard often enough about people pushing cars over 1.000.000 kilometers. And the story of those people were almost always the same. The gist is/was: no wild driving and proper maintenance at very regular intervals, using only manufacturer/factory approved (engine) parts and lubricants.
Of course, spending a lot of money on an old(er) car to keep it running reliably or buying/trading in a car every few years likely won't matter much, cost-wise. Unless you are capable (know-how and having the necessary tools & equipment), which reduces incurred costs significantly. Anyhow, people will choose the latter way, just because they're addicted to the new car smell.
Ten years ago I had an 15+ year old Peugot 205D (the cheapest, most spartan model). It had a very basic, old-fashioned diesel engine and on average (including highways) it would consume 1 liter of diesel every 20 kilometer. If I skipped the highways and made an effort I could manage 25 kilometers per liter. And that with an engine that had already 300.000+ kilometers on the clock.
In my experience, diesels today, with common rail injection and what not, barely go 15 kilometers per liter, even when you make an effort. At least in Europe you hardly notice the difference between having a diesel or standard fuel engine in your car anymore. Road taxes, fuel prices and trade-in values are still quite different.
TL:DR
New cars are hardly better at anything than older cars. Other than giving the consumer a (false) sense of safety that is.
