I think it’s fine to say or write -es or -I in general usage. I think it’s also fine to prefer one or the other in contextual usage. And I think it’s fair to roll ones eyes at people making a big deal of it.

But it’s incorrect to say octopus is not a Latin word. That’s like saying utensil isn’t a French word. It is, and it came to English from Latin through French. Octopus came to English through Latin from Greek. It’s a Latin word and it’s a Greek word.

So, I don’t think it’s a consistent position to be okay with the Englishification of Latin words, but not be okay at the Latinization of Greek ones!