I've also heard something like this: If you put a new 'top end' on an old engine, the 'bottom end' won't last.

I think it probably arose because someone took an old engine that needed a full rebuild, and ONLY did head and cam work on it. Then, they run it hard because of the new power that they've got. The engine starts to knock, maybe because they put a much bigger cam in it and head(s) with much bigger valves which let in a lot more fuel and make more power, or probably just because it was going to start knocking soon anyway.

I replaced the head gasket on my 22RE 10 years and 90,000 miles ago when I bought it with a broken timing chain. I just did the head gasket again (and a new head and cam) because of a burned valve. I elected not to mess with the bottom end because it isn't knocking, and the other 3 cylinders (without burned valves) had really good compression.

Hopefully I won't be worrying about it for another 90,000 or so.