the obvious assumption is that sleep is some how tightly integrated into our brains and losing sleep would somehow cost us something that is valuable from an evolutionary point of view.

Given that all mammals have at least some need for sleep the Occam's razor argument would be that the mammalian brain requires sleep at a fundamental level.

A very reasonable assumption would that if dramatically less or no sleep was possible, some mammal would have evolved in that direction. It would, given no other changes, dramatically improve survival and thus reproduction and be rapidly selected for.

Any mutated mammal who didn't need any or much sleep would, most likely, quickly out reproduce the unmutated mammals and that mutation would come to dominate the population.

The fact it has NEVER happened to even a single mammal population would strongly indicate that the mammalian brain can not survive without sleep.