Ever replace a bike due to mileage/use?
Yes. A ~1990 sport Peugeot I bought for $20 that had been hit hard by an SUV. (Trashed fork. Dented top tube. Both chainstays cracked and about to break. Rode it fix gear 8,000 miles but stayed out of the hills for fear of breaking it. Loved the ride so much I had a custom built so I could take that ride into the hills. When it arrived, the Peugeot was retired.
That is exactly why all my bikes (and forks) will be steel or titanium. Far less likelihood of the sudden failure common to both carbon fiber and aluminum. I've ridden several frames with multiple crashes on them for 20,000 miles or further.
And yes, I have had several steel frames break on me but those breaks have never been catastrophic. In fact, non were even ride enders (though I did drop out of a race for one of them. Rode easy back to the finish.)
Ben