>>Few atheists believe that life is meaningless.<<
I agree. But that is because I believe that few atheists are really atheists.
>>They might reasonably ask you why you are so convinced that life without your version of "God" is meaningless.<<
That is not what I believe.
Hard to tell that from your post:
For without God, life truly has no meaning whatsoever. Even the extermination of all mankind - or it's proliferation, would be totally irrelevant.