It can be viable to buy a billy and sell him on after a couple of years when his daughters are coming through.
Our last boy was about 5 when we bought him as a proven male, and we sold him on a year and a bit later after two mating seasons for stud use.
We've replaced him this time with a yearling buck who is from good lines and with cracking confirmation, so would expect him to sell on well in a couple years when his daughters have come through.
I expect most boys will get sold around like this for a while, and after a time, things like bad feet, arthritis, even infertility will make them less useful. Some may send to the knacker man, but there is a market for older goats for meat, and outside breeding season there wouldn't be the same taint. If you put them through the market at that point, someone would snap them up for meat if you didn't fancy it yourself.