Bucking horses are a pretty big business now a days. And as stated above, some wind up back in the breeding program, and others sold.
I've heard of buckers competing well into their teens. One horse at a rodeo I went to this year was 19!!! He ditched his rider too, so he was good at his job for an older horse!
I would say that many of the buckers, after doing it for so long may never be retrained to actually ride; I think the tendency to go back to their 'old job' may be too strong. This is not to say that "all" buckers are hopeless causes that couldn't be retrained to ride, or perhaps drive, but you may not be as successful with a bronc, as opposed to an OTTB.