I've retired bikes for changes in technology, not age, condition, or mileage.
The last one was my 1996 Novarra Randonee, after 22 years of use, guessing 3-5000 miles/year (touring, commuting, errands and fun rides), so definitely over 50K, maybe 100K miles. There was absolutely nothing mechanically wrong with it, but I replaced it with a disk brake bike because I was getting tired of the fussy old cantis. I must have adjusted those at least four times a year, close to 100 times.
I got the Randonee because it was on sale and had brifters, which was a new tech on production bikes back then. Before that, it was for indexed downtube shifting in the '80s.
I had new one bike stolen in the early 80s (a wedding present), but the replacement was almost identical. Luckily I'm with the same spouse, though.