Tell that to the all the ~80s w118's etc. around the world still working as taxis and running off vegetable oil

the 80s started 40 years ago.

If my calculations are correct, 40 years is actually less than always

This comment doesn’t say anything about the maintenance/repair cost to keep those cars running.

They're (were?) a popular choice in very poor countries due to their reliability. They did used to be built to a higher standard. The story I heard which I have no idea about the truth of, is that competition from lexus coming along in the 2000s made merc more price conscious, and reliability suffered as a result.

That's terribly untrue (or relates to particular market). Mercedes of 80-20x were one of most reliable on market, but unfortunately quality tumbled sharply.

I have driven lots of them in 90s-20s and thought it were the best cars. And it were.

My old W126 was an amazingly well built car. Very durable and reliable, extremely long lasting, very comfy, interesting to drive, and amazingly safe compared to other cars of that era. Mine was previously owned by a couple who just did bare-minimum maintenance and drove it 120k km over their 6 years of ownership. The car was 24 years old when they got it, and 30 years old when I bought it. I drove it many more years, and the only repairs I needed to do were fixing age related leaks, and replacing one relay.

The W126 was a fairly simple car though, compared to their modern offerings, and it was built under a philosophy of building the best car regardless of cost.

My W140 was a also a well made car, great quality in materials and mechanical fundamentals, but it was too complicated and quite a bit more problematic for me.

Modern Mercedes-Benz cars don't seem as distinguished or well built as their offerings from the 1980s. With that said, my C216 CL-class is a very well built car in terms of materials and general attention to detail; it's just too complicated with hurts its overall reliability (though the powertrain is decently solid).