Detroit's are "forever engines"?

I am considering buying a rebuilt Detroit 71 series generator. I was told that they are "forever engines" because the liners can always be replaced to bring them back to stock specification. I also have seen some 50 year old engines that are still in daily use. Some of the things that I have read to the contrary is that the heat cycles eventually cause the block itself to crack in many places. Which is true? Do blocks just eventually break down, or can these engines be run continuously, even after 6, 7 rebuilds over the years?

Back 40 years ago when I went to Diesel school the instructors said as long as the block was ok you could put in oversize liners & pistons in and rebuild it.