Why is that silly? The work involved to remove and rebuild the engine in a large piece of earthmoving equipment is far greater than the labor to do the same in the average farm tractor. Too, back then if a part wasn't worn out the idea was to reuse what parts you could and only replace the parts that were worn beyond factory specs. CAT, for one, has always had a guide showing what was acceptable and what wasn't, along with the standard 'out of service' limits on their pieces. That said if you tore an engine down, say in an old D9, and the liners weren't worn beyond specs, why replace them? In a case like that what you do is hone the liners to get them ready for a new set of rings, clean the pistons (if they aren't damaged), install new rings, and put it all back together.
Regardless of the engine size, what it sll comes down to is exactly what I said before, why replace parts that aren't worn out. Unfortunately, nowdays alot of the thought on reuse vs replace has to do with the cost of labor. In other words the new part is cheaper than the cost of the labor involved to clean and reuse the old ones. In a case like that, then yes, new parts will always be used. Thing is when it comes to the large engines that isn't always the case and the reuse of 6 $500 liners, that only take $120 each in time and labor to reinstall with new seal rings and hone, actually makes good sense.
In the end I'm all for using new parts in a rebuild when necessary, but when the old parts aren't damaged and are well within spec, why waste a customers money when youm don't need to? In fact I'm currently having to do this on a 6 BT Cummins because a customer decided not to go with a reman engine, against my recommendation, and instead went for a repair. They had gotten someone else involved in the 'repair' and I got called in a month later to complete the deal. Now I've got to 'repair' the repair, which has already cost them close to $4000 and do it within their budget. One cylinder has already been bored and sleeved to standard and has a new piston, but to get them back and running now means pulling the other 5 pistons (that had been allowed to set with water on top of them for several days), replacing one of the pistons that has pieces of debris imbedded in the crown, honing the cylinder walls, and reringing the remaining 4 pistons and reinstalling them. While this line of "repair" grates against every bone in my body, but it is the only way to get this engine running again short of throwing the money already spent away and starting over. True it's still got old bearings, seals, etc, but what will be done is as close to doing an 'inframe' rebuild as I can get them, and at this time, unfortunately, it's about their only choice. However with the 'used' cylinder walls properly honed, the old, undamaged pistons, and a new set of rings it's really no different than the conditions you'd find in new/reman engine.