All my cats were 'rescue' cats, apart from my female, but I can understand about the rehoming side, and how much life is still left in a retired breeding cat. My oldest cat came to me at 6 and he's now 19, so I've had 13 years from him so far, infact he's sat on my knee asleep now.
I've know several cat breeders who passed on their retired cats and they rehomed them in a very similar way to their kittens, and yes they had a hard time when they let one of their favourite ones go, but it was kinder to let them be rehomed than to keep miserable cats who had to be separated because of the other breeding cats.