When randomly removing trait cards, luck plays a bigger role. What if - in an extreme case - only 1 person picks up a carnivore card? Etc. By replacing 1 trait in its entirety, everyone has the same chance of picking it up.
In a two player game you remove 40 cards. So if you add say Gigantism to a two player base game, you just remove 47 cards. If the randomness is okay for a two player game what is wrong with my suggestion.