In the lower liner bores for smaller erosion problems we had a procedure to use Belzona then remachine the lower line bore, this repair usually lasted until the next major overhaul, 30-50,000 hrs. More severely damaged lower bores were cut oversized and sleeves were fitted, we had a number of engines operating with reworked lower liner bores for an additional 100,000 hrs.
I recently did a plant upgrade on older natural gas engines that had been in service since 1983, with an average annual runtime of 8400 hrs/year, so each engine had about 268,000 hrs. Those engines had top end overhauls every year-cylinder heads, turbo's, fuel system repairs, about every three years major overhauls (what we called in frames), that added liners and rings, and crank bearings. Each engine had two shop overhauls down to bare block, that included machining the counterbores, top decks, and line bores in their life. Those engines ran 100% of their rated load all the time.
I think the longest running plant I worked on was in Australia, with the engines installed in 1947, they were gradually replaced from 2009 to 2012, the oldest engine had about 425,000 hours on it, as I remember those engines averaged about 6500 hrs/year at an average load factor of about 60%.
I'm sure someone else here has had contact with longer running engines, I know some manufacturers have cited 500,000 hours on some of their engines. So well built properly maintained heavy duty engines can last many years and rebuilt many times, virtually all I know of using wet liner type technology. Some industrial diesels did use dry sleeves but usually higher speed smaller units used in boats and standby generators.
We used to use a formula that CAT came up with for "equivalent miles" I'm sure some sales guy thought it up for our engines that were used in both industrial and on hiway trucks, they used 1 engine hour equaled 40 miles, which actually seemed to work out pretty close for one of our most popular models. So if I apply that to my two old gassers 268,000 X 40 = 10,720,000 miles (at or near full load all the time).
Not really a good comparison to your car engine, but wet liner technology has been around a long time and used in engines that easily go beyond your 3 million mark all the time. Like everything else, I'd say it's not just the liner, but how well the block supports the liner and other moving parts (like the crank) that provides the overall longevity to the engine.
MikeL.