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.