We are basically commercial, but mostly buy purebred bulls to use. We will keep one we like, like his calves, dispositions, all that, until he is either shooting blanks or gets to where he won't stay home or whatever. Finally sold our Red Poll at about 14 as the arthritis in the previously fractured hock joint was just making it soo hard for him to get up and down, and we didn't want him to suffer through a cold winter and not be able to get up. Have 2 bulls that are 9 and 10, still siring good calves, both are easy calving bulls and we still use them on heifers. They are gentle breeders, and no problems with dispositions. Have not had them ride a heifer down yet. Sometimes they are used back on their daughters, but try not to. We seldom keep heifers out of them, but there are a few that we have. I can rotate them back and forth so that they don't often get their daughters to breed.
We have several that are "cow bulls" .... not easy calving. Use them on the older cows. Keep a couple to rotate out so that they don't all get to work every season. We calve twice a year so they get chances to work and chances to rest.
Have another that is 8 now and a very good breeder. Like his calves. He gets his girls taken care of, but then he goes to the neighbors girls. So then he comes home to the "bull lot"....Have kept several of his heifers, and have 2 bull calves we have not banded yet.... don't usually raise replacement bulls, but like you, we like these 2 calves alot.
Since we rent several places, I can be careful of breeding too close.... but there are a few retained heifers, now cows, that are sired and then great grandsired by the same bull.
If he's a good one, he stays as long as he does his job and minds his manners....