This would be great news if the whale literally only required krill to survive, but complex megafauna have complex needs, so the ability of krill and other small creatures to evolve is largely irrelevant in a discussion regarding the ability of megafauna to survive. This is especially true if you read TFA and see that the whales already adapt to eat different things as necessary.

Humpbacks have a highly specialized feeding mechanism. They only prey on krill and small fish.

The food chain really is sun -> algae -> krill (and sometimes small fish) -> humpback whale