Because evolution is slow and the climate change is going fast.
The food chain really is sun -> algae -> krill (and sometimes small fish) -> humpback whale
But they don’t eat large fish, squid regularly, or anything like seals - so they’re not “generalists” in the broad, anything-goes sense.
They’re still constrained by their baleen filter-feeding system, which limits them to tiny prey
Lots of things eat krill and small fish. They're near the bottom of the foodchain too. In addition to algae, krill are opportunistic omnivores who often consume detritus. But their primary diet is algae. Small fish tend to be pretty similar.
It's not that other things don't interact with algae or krill or small fish, it's that those groups are the foundation bedrock of the ocean ecology. And single celled organisms like algae are tough as nails in aggregate. Couldn't kill them all if we tried. Pool owners will be familiar with the struggle.
A perfect example of this is when sea otters were nearly hunted to extinction which caused sea urchins to flourish which caused the death of coral and coastal environments which started to affect the larger things that depended on those environments.
My point is that any change to the careful balance can have non-linear effects.
Like what? Emotional support dolphin?
But seriously, we may not have much choice. Natural evolution stopped being able to adapt to environmental changes after it created us; genetic engineering is essentially the only way to make biology adaptable enough again.
But since we humans are pretty arrogant with our wisdom and lack long term patience, I can see many ways where well-intended meddling can end up in catastrophe overall.