The offensive initiated by the black mechs had never been about wiping the Violet Estates from the map.
Their overriding mission had always been to crush the members of the Purnesse Family!
Once the Larkinson Clan recognized this truth and moved to evacuate the trapped Purnessers, the black mechs went crazy!
They no longer played cat and mouse against the Avatar and Vandal mechs flying in the air. Instead, they swam around the island and tried to throw as many attacks towards the departing combat carriers as possible!
Kinetic shells kept bouncing against the hulls and vulnerable components of the escaping vessels.
The attacks exerted a considerable degree of pressure towards the combat carriers.
If the six ships were still in their prime, then they could have easily resisted the attacks for a time.