That is bad logic. It means that it could be less frequently used for the meaning you would like. However we have no evidence of that. No written term of atheos meant that prior to the first century bc. And thereafter its use included agnostics. I do not believe it was used prior to 500ad in the manner for which you are pushing. If you have an example cite and show it.
I think that the question is whether it should include people who are incapable of belief. I agree that it has come to include this meaning for many, I do not think there is any reason for this. We have two categories of people. The first is people who have never heard or concieved of a god, the second are people who have heard of and concieved a god. The former lack belief in a god, the latter are agnostics.
But, according to your logic, you are using the term "agnostic" incorrectly. It was originally used to describe people who thought that knowledge of god was impossible, not those who neither believed nor disbelieved in God or gods.
To me, the most accurate, reasonable way to define the term "atheism" is merely the absence of "theism". Theism is an extremely general term with many subcategories. Atheism is the same. It has many different subcategories. There are theists and atheists. That's it. There isn't a need for anything more at the top ... that is what the subcategories are for.
It seems that the only ones who are against this use of the term atheism as a general term are the frustrated religious who want to use atheism as some kind of derogatory term. One member even went so far as to say that, because I am saying that babies are technically atheist according to the definition of the term, I am somehow sentencing babies to an eternity in hell. Anyone who sees atheism as some kind of negative, damning label is far too biased to have a valid opinion in this conversation.
So, I ask you. Is atheism a negative thing?