I have four cars that were all built in the 21st century. All of them have over 100,000 miles, one of them has over $230,000 mi and two of them have over 180,000 miles on them.
I was asking a mechanic one day why is it that engines last so long nowadays and his response was something like this: remember back in the day when car enthusiasts would blueprint their engines? Nowadays all engines are blueprinted at the factory.
There may be some truth to what your mechanic said, however my opinion is different.
My opinion is better fuels and oils.
The leaded fuels of yesteryear really made hard carbon deposits in the cylinder, of which numerous particles made it past the rings and into the oil. These very hard particles acted like microscopic sand paper that constantly wore down the rings, cylinder walls and bearings.
Lead free gas leaves a black sooty deposit in the combustion chamber and is practically non-abrasive.
By removing that “sand paper” along with modern oils, I think the engine life is about 3 times that of a few decades ago.