why the exponent is three
A linear lock still pays the first wallet in: buy at the open and wait a day, buy at half and wait twelve hours, and the cheap tokens are worth the wait either way. Cubing what is left of the curve collapses the middle instead. Half the curve costs three hours. Seventy per cent costs thirty-nine minutes. Ninety costs five.
So the cost falls on the wallet that lands in the first block and almost nowhere else, which is the only place it was ever meant to fall.