You know, words do change meaning. Hell, they do so on a daily basis. There are people whose only job is to track the usage of words to determine how they're being used, and update their definitions in dictionaries. Linguistics is a very real field.
Besides, you're assuming the word was made when we knew exactly what the Earth was. The word, or to be more correct the ancient form of the word (Isn't it Gaea, or somesuch? I know that's the Greek goddess of the Earth, in any event), was made to describe what we knew, not exactly what it was. Because we didn't know exactly what it was, only the part of it we could observe. Hell, they used to think the Earth was the center of the universe, and that was proven wrong (Unless you take into consideration that we are, indeed, the center of the observable universe. But that's a different matter). They also used to think germs were evil spirits, or tainted blood that needed to be let out.
Your entire argument has no basis at all, really. Hell, you don't even use the right definition of Earth, nor do you bother to do the research to figure out what the Earth was first called, as that would be more pertinent to your point. Next time, do your research before making a thread.