I've only gotten rid of one bike. It was about 23 years old. The wheels and the drivetrain were shot to the point it was no longer safe to ride and totally not worth the cost of repairing. With three other bikes of my own and the rest of the families' in the garage it was better to make a little room than hang on to a useless bike for nostaligia, but I felt bad seeing it go.
But I think with, say, aluminum entry-level and near entry level road bikes 10 to 20k, with high end bikes keep them forever unless they become unsafe.