Nobody? GP said "While every animal seems to eventually become like a crab...". That's not nobody. I think you'll find a lot of people online who have gotten the "crustacean carcinisation" idea wrong and think it applies to all animals.
Nobody in the scientific community is saying that fish keep evolving into crabs.
Nobody in the scientific community except humans is saying that fish keep evolving into crabs.
Source: Pedant.