I am interested in how long the sex-links live, too. I order sex-links to fill up the box when I get chicks from a hatchery. Beginner chicken keepers do not want a chance rooster, even though yours worked out. Kids will get attached to the male chicks every time! Too many city laws against roosters.
I wonder if a local breeder with heritage breeds might make a longer lived sex link than the hatchery produced birds. A heritage breed like a Bared Plymouth Rock and Rhode Island Red cross is likely to have hybrid vigor and maybe a longer life, but probably not the excellent egg production of commercial sex-linked hybrids. It is a question that the more experienced breeders on here maybe could answer.