> boom-bust cycles enable capital to buy everything in the bust period.

Out of curiosity, what happens to the busted capital in this toy model?

The capital in monetary terms is destroyed, it's then either bought out by surviving capital and concentrated in actual terms, or destroyed. In either case, the relative concentration of capital increases.

Often it gets bought and consolidated into a bigger entity. In the 80s and 90s it would be shipped to Mexico, Costa Rica or (in later years) China where it was put to use operated by a much cheaper workforce.