You can can absolutely breed for traits like litter size and adult size without completely homogenizing your mouse colony through inbreeding. Pretty much anyone who breeds feeders is probably doing that by default and getting rid of poor breeders, slow growers, etc and not keeping them as part of their breeding program. You can apply selective pressure to a species without making them fully inbred. I was looking up mouse lifespans and it seems like in the wild they can live up to five years, although it’s usually less due to predation, and ~2-3 years as pets. Meanwhile I can assure you there are no mice in my (laboratory) colonies that are making it to 2 years. I usually euthanize at the 9months-1yr mark for my breeders because they get old, and frail and sad
Anyone who has ever lost a ball python in their house can easily attest that these guys can really go the distance if they want to. While I have no doubt that there is inbreeding occurring in the wild, there is plenty of opportunity for genetic diversity. Breeding back to a sibling/parent here and there is very different than perpetually linebreeding to a high degree of consanguinity. Biologically, genetic diversity is in a species best interest.