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I am considering the possibility of having massive cephalopods evolving on my world and having them fill the niche of whales.

I've heard about how whales have evolved to increase in size for the purpose of obtaining more food. I've also read about the giant and colossal squid, so it doesn't seem too outlandish for a squid to evolve to the size of whales. The reason why whales can grow to such ridiculous sizes is that they don't have to support their own weight.

For locomotion, I'm thinking of elongating their mantles to allow them to swim in the same manner as whales.

As for feeding, I'm considering two main ideas. They are able to repurpose their siphon to take in water, and then keep some sort of baleen like mechanism that filters out krill, small fish, or whatever else is small and form schools on my world. My other idea is that they have webbed arms that close onto food and guide it into their beaks, which have been redesigned as a mechanism to trap large amounts of prey.

They play an important role in the ecosystem, similar to that of whales, but more stable. They reproduce faster and are more fertile when young. Instead of birthing single helpless calves, the create long chains of eggs. Whaling would be more sustainable on these creatures. They are fed upon by sharks and other smaller predators, as well as larger sea monsters. Their ability to consume smaller creatures lower on the food chain makes them more numerous and thus a more sustainable source of food.

Does anything about this sound implausible? Let me know.

I see nothing inherently wrong with this idea. In the absence of whales and other sea mammals something would fill the niche. Prior to mammals there were very similar reptiles, so why not squid.

https://www.livescience.com/54870-giant-squid-may-be-school-bus-size.html suggests that existing giant squid could reach 20 meters in length. Given a lack of whales and the biological variations you've listed it seems entirely plausible for squid to reach the sizes you describe.