of course steel will fatigue and crack if its endurance limit is exceeded - the point is, it has one.

There are limits to design that can affect failures, and that envelope was often pushed for light weight.
There is also mechanical damage, wear, contact stress, binding, etc. that can increase operating stress above the endurance limit.

For an example of endurance I can throw out a 40,000-mi frame and 25,000-mi Zeus axle - I bought them both new.
I wasn't even worried cold-setting the rear triangles an extra 6mm. I wouldn't even think about setting it back.

There are defects in brazes that can eventually catch up. As mentioned before, once you add corrosion damage, all bets are off.
But again, we're dealing with a mature technology