> How do the gears of capitalism keep turning without UBI in such a scenario?

The part you've got wrong here is thinking that Capitalism is equivalent to innovation and growth, which it is not. Capitalism simply serves capital, and with less and less of its constraints being physically bound to resources, there is very much a plausible world in which only a small handful of consolidated bank accounts exist so long as those bank accounts can reasonably control the monopoly of violence.

There is a point historically in which Capitalism absolutely served to provide widespread innovation and prosperity at scale, but this is not a feature of capitalism inherently but rather a consequence of it when run within the existing system of labor mechanics at the time. Capitalism can still very much exist within a Feudal structure of governance, its contributions to society will be much lesser, but it can still very much exist.