“Octopi” is shorter and easier to say than “octopodes” and it’s consistent with most other loan words in English that end in -us. Who cares if it’s not correct Ancient Greek? We’re not speaking Ancient Greek, we’re speaking English. Substituting a second-declension Latin ending for a third-declension Greek ending is the least of this language’s problems.
Anyway, I think we all know that the correct plural of “octopus” is “hentai.”