Historically and economically, it is far from certain that your hypothetical 20% increase in productivity would actually result in a proportionate decrease in employment. Indeed, the opposite effect is sometimes observed. Increased efficiency makes each employee more productive/valuable, which in turn makes newer and harder problems cost-effective to solve.
Personally, I question whether any AI coding experiment I have yet performed myself resulted in as much as a 20% productivity gain anyway. I have seen plenty of first-hand evidence to support the theory that seems to be shared by most of the senior+ devs I've talked with, that AI code generators are basically performing on the level of a broadly- but shallowly-experienced junior dev and not showing much qualitative improvement over time.
Whenever yet another tech CEO trots out some random stat about how AI is now writing 105% of the new code in their org, I am reminded of the observation by another former tech CEO, Bill Gates, that measuring programming progress by lines of code is like measuring aircraft building progress by weight.