There is no public info on the technical aspects of the side-scan sonar search, e,g., at what depth was the sonar operating. There are military side-scan sonars that can operate at pressures up to 3000 meters deep, so perhaps if one of those were made available, they might re-do the search grids if the sonar during the second phase search was operated at a much higher depth..
Another side-scan search might find the main wreckage. Even if the main wreckage is found, the odds of finding the boxes are pretty low. If the fuselage slid down the side of a seamount, bringing down a seaslide of sediment and rocks with it, the boxes might be buried.