Alr simple, when the craft that is taking a region gets sunk, make the reason immidiatelly end, so for example a person with a siege base 2 chunks from the region can't use multiple tanks against like 4 tanks to take the region, this might possibly cause people to siege earlier so incase they fail, they can attack again.
before:
when the attacking craft sinks, the region ends within 10 minutes or so, giving them time to bring another craft and win it pretty much with cringe tactics.
after:
when the attacking craft sinks, the region immidiatelly ends, forcing the attackers to give up or bring another craft to resiege it, which gives another 10m for defenders to resupply etc, also people might siege sooner so incase they sink they could siege again