In the old days, things were built to last forever. There were parts that wore out, but they were very small and replacements were cheap. Things like light bulbs, bearings and brake linings. However, the down side of many of these parts is that it took considerable time and effort to replace them.
Nowadays there are no small parts. Some little dohickey fails, like the door latch, and you don’t replace just the failed part, you replace the entire subassembly, like the entire door.
On my car, anyway, the latch and the striker are individually replaceable, though the prices will make your nose, or at least my Visa, bleed. (Still, gotta be cheaper than a whole door, right?)
The smallest part I can remember replacing is the cap for the coolant-overflow tank, which was priced at a startling $10.21, more than an oil filter but way less than a wiper blade.
On the other hand, parts last a lot longer now. I got 95,000 miles out of the brakes on my truck before they had to be redone. Actually, I think I still have the original brakes on the rear, and I’m up to something like 110K miles. Engines used to last 100,000 miles, now they should last 250,000. However, transmissions which used to be good for the life of the car now seem to be a weak link.
I suspect that fiendish complexity is responsible for (some of) the seeming weakness in slushboxes these days. The old Chrysler TorqueFlite A727, introduced back in the early 1960s, was just this side of indestructible, and could be adjusted by that kid who worked weekends at the Texaco. Get one of these modern-day six-speeders (or seven or eight) and look at it crossways, and suddenly you’re looking at a $3000 rebuild.
Then again, nobody seems to change transmission fluid anymore. And it’s not like they make it easy to do it, either. In my previous car, you couldn’t even drop the pan and drain it: the pan was on the side of the case. Being resistant to the whole idea of $3000 rebuilds, I change out the stuff every 30k or so.