Well, with the example you gave, both “fish” and “fishes” are correct. Fish can refer to a plural form of one type of fish (“I went salmon fishing and caught a lot of fish”), but “fishes” is the plural form or many different kinds of fish (“There are many fishes in the ocean”). “Fishes” or “crabs” didn’t arise because of misuse, they are words in themselves. Alright, rant aside. I agree with you, to a point. Language does evolve, so there are many instances where terms and grammar may be correct, but were not so 100 years ago. But there are some things I can never accept as “acceptable”. For example: disorientated. That’s not a word. I don’t care if it is in dictionaries, it is awful.