Mileage is much less important nowadays than it used to be, particularly with diesels. Because diesel fuel explodes, where petrol burns, the engines need to be built more robustly and last longer. On top of that because the power develops at lower revs the engine is turning over more slowly than a petrol engine for a given road speed, so wear is reduced there as well. Engines used to be getting worn out at 100,000 miles but my last two cars, both diesels, have both done well over 150,000 with no obvious signs of wear and the current one is close to 200,000 and is booked to take my caravan into mainland Europe this summer. You need to be doing 10,000 miles a year for a diesel to be cost effective but just take a look at the websites of the likes of Volvo/Jaguar etc and you'll see a predominance of diesel engines for their new cars; not what you expect for a fuel on its way out.