I suppose some military campaigns do end in a failure, like this one. It's just that cost is astronomical for humans. They talk about the ink clan's losses, but that barely matters: the ink clan grows in numbers by mass production. Since they didn't destroy the royal nest, the whole network of territorial and feudal nests remain as well. Nobody was going around destroying those since the plan was to destroy the king's nest, which would have withered everything below it as well. With existing, fully developed nests ready and waiting, new feudal and territory ink nobles will sprout in no time. But how about all the human losses? It's extremely difficult to compensate for those.
Even if they capture the Evolution Pass fortress, this whole operation will be studied later as mostly a failure, to plan any later operations much better.