An economy can keep running even if most people are cut out of it. Production would just reorient to who owns capital, the state, and other AI systems buying from each other.

But, if I get the insinuation, this would still require a huge social change to absorb once broad consumer demand falls off.

As for pursuing math for fun: that would turn most disciplines into something akin to speedrunning, chess, or mountaineering. People would add constraints to make it harder for themselves and compete against each other, and that does motivate some - but not all. A lot of people want to be "on the frontier." If discovery gets automated, the remaining frontier may be physical, actually going places no one has been, and that might pull those people toward space as it opens up.