> Those 20% would just figure that there's no need to have children when you can just focus on yourself and go largely extinct within a generation, replaced with some people from the higher tiers, but not nearly enough to make up for those who weren't born.

A person being _unemployed_ is not a hereditary trait. Also, while it's already pretty wild to effectively claim that about 20% of humanity carries some kind _doomed to be made unemployable by AI_ gene, it's another giant leap beyond that to declaring this cohort will uniformly choose not to reproduce and therefore become extinct.