Here's one anecdotal "test case", if anyone's interested. This is my F250 I use to haul equipment, gravel, mulch and such.
I've been running a 7.3 IDI in my truck I bought around 2000, it's a 1988 model. Never seen 100,000 miles, but it's seen 99,999 about 5 times now (5 digit odo). The most money I've ever spent was on upgrades - Banks Turbocharger Kit(2K), Pump and Injectors ($800). If block worm ever takes out a cylinder, I can get a prepped block and rebuild it for less than 2K, or buy a parts truck or two for less than 3K. I do all the basics that someone keeping a vehicle for the long haul does, upgraded fuel filtration system, bypass oil filter, lubricity additive in the tank, preventive replacement of oil cooler o-rings, new radiator, and gauges for oil and water temp and oil pressure (factory idiot gauges are useless). That along with oil and coolant analysis has kept it running fine with no major unexpected expenses. It would last a lot longer if I didn't put the turbo on, probably.
There's only a few things you need to watch on the old 7.3, and that's for poor starts, dripping, stuck or bad pattern injectors (smoke on start, slight misfires, roughness at any RPM), SCA levels in the coolant, and oil contamination from the cooler on older trucks. As long as you don't run it with any of those problems, it's usually in for the long haul.
I've put a lot of money into it over time, but that would be true for any vehicle I've ever owned - preventative maintenance and upgrades aren't cheap - but they are a heck of a lot cheaper than a new truck. I haven't spent half what one new truck would cost and this thing is going on 30 years old. In fact, on the "normal" 3-5 year warranty expired buy a new vehicle treadmill many people are on, I could probably rebuild the engine every other year and come out ahead vs. buy a new truck with a warranty each time it expires. But, my paint isn't as shiny as it once was.
