The damage done by doomscrolling and social media goes far beyond the "development of the brain for reasons of economic participation". Also, you seem to think the development of the brain doesn't have any purpose beyond that - a view I don't share.

If - and that if is doing some heavy lifting; given the underlying, fundamental problems of LLMs it doesn't make any sense to assume that we'll get there anytime soon - LLM AIs will be able to dominate economy in such a way that humans cannot compete anymore, that simply means that humans will have tons of free time at their hands. ( There's a reason for the grain dole that the Roman elites handed out to the poor of the city. I assume if you have a whole population that doesn't have anything valuable to contribute to the economy, the economy will still have to find a way to sustain them. Otherwise riots.)

A well developed brain seems to be exactly what I would want in such a case.