They increased the size of the diameter of the guide rod assembly when they went from the gen3 to the gen4. This made the hole in the front of the slide larger/weaker. It takes no where near 100,000 rounds for the gen4 slides to crack. Not internet lore, common knowledge, start looking for cracks at the 20,000 round count.
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This is why I asked if the beefed up the slide on the gen5's.
20,000 rounds sounds like a lot but if do any shooting at all, you'll go thru 20,000 rounds in a hurry.