My guess is that all rotating engine parts are designed not to flex enough(in normal use) to really have a fatigue life, they are usually used below the fatigue limit, so the number of stress cycles is irrelevant.
Now say if you have a bearing with too much wear and things start banging around or flexing, then things could start getting stressed above the fatigue limit and then the number of cycles and forces starts to matter before something breaks.
I would think even valve springs are usually designed to not exceed their fatigue limit, so in theory they could go forever too? Throw in too high a lift cam, then the stock springs could be used over the fatigue limit and break after X cycles.
All this assumes nearly perfect parts, so if one con rod has some impurity inclusions, that one may be fatiguing, but with so many miles on it, probably one of those would of failed already.
So probably its a good idea to stress an engine significantly while its under warranty to find any defects like this, and then out of warranty, be a bit more gentle.
