My current cars don't have very high mileage but they are getting quite old. My daily drivers are a 19 years old Toyota 4x4 truck with 171,000 miles and a very similar 22 years old Toyota with almost the same mileage (271,000 km). I got both of them used 3 years ago with 124,000 miles on the 1993 it and it's been quite reliable since. I replaced the timing belt on the V6 at 130,000 for the first time and did the valve cover gaskets and replaced the original radiator at the same time but it's been pretty trouble free. The a/c and every accessory still works. Even the power antenna does! The 1990 Toyota has no power options or A/C and I replaced it's 4cyl engine with a better one that I already had a few years ago at 257,000 km and I had to repair some rust on it's body since it spent all it's life in Canada. Next spring, I should replace both box sides as I have new ones waiting to be installed.
My older cars have lower mileage than that but I had to replace the 430 engine on my '67 Buick Riviera at 74,000 miles after mine self-destructed on the highway. I wanted to go to the service station at the next exit and have the car towed from there but I didn't make it, even after slowing down! Just a minute after it started to knock, the bottom end exploded and a rod went out with a big chunk of the block and it even did a small hole the frame where it punched it! The only clue that something was bad before that day was the low oil pressure reading on the gauge and a few seconds of bearing noise when I started the engine. And I had a similar Buick 455 (which I thought was worse than the 430 when the 430 exploded!) rebuilt at 85,000 miles on my '75 Electra as I felt the same was going to happen soon! I was aware of this before I got the car with 64,000 miles on it as the oil pressure switch didn't work when I went to test drive it. I asked to the owner if I could replace it before buying the car and when I did, the oil light came on at idle as soon as the engine was getting warm!
I still managed to do 21,000 miles with this tired engine after I negotiated the price of the car 1500$ below the asking price.
The original engine and transmission are still fine in my '65 Buick which now has a bit more than 100,000 miles on it. On these cars, I think that age, lack of maintenance and long storage periods did more damage than the mileage. My '75 Buick had been stored from 1981 to 2001 and I'm wondering if they did a single oil change during these 20 years. But the Buick 350-430-455 and early V6 engines were clearly not as good as the older Buick Nailhead engines or even newer ones (the V6 got much better in the late eighties but I still had to replace a 1991 3.8 V6 at 145,000 miles when it's bearings went bad).
I think that more precise fuel systems, better design, alloys and other improvements like roller lifters and rockers really made most newer engines much better. The newest ones even work with 0W20 and 5W20 oil in hot weather. But I don't think that transmissions got that much better than older ones. They are now more efficient but I doubt they are more solid than a GM ST 400/TH 400 from the 1960's.