I like to think that, especially if we lived for millennia or even eternity, we would especially like to make that infinity of time as pleasant for ourselves as possible. We'd still want houses and comfortable beds and ready supplies of food and something with which to power everything. I can't imagine being willing to live forever as mere animals. I couldn't do it.
I'm also an atheist. But I recognize the human drive towards truth and some kind of meaning in the universe. That's the basic impulse underlying existentialist philosophy (which I subscribe to). I've been an existentialist since I was 17, and the past 50 years have been a search for meaning. I don't think I would lose that even if I lived on Earth forever.
The problem as I see it (and this may actually be what you meant) is that postulating an afterlife may dampen that search and its attendant drive to succeed and create. After all, if this life is merely a stepping stone to what is supposedly the purpose of existing, our "real" lives as it were, then there is no reason to put any stock in improving ourselves here and now. And if, in addition to this, we are going to be "forgiven" for our behavior here, then what is the point of improving that behavior? Looking at the actions of those who so believe, it is obvious to me that they think there is no such point. They exchange their humanity for angelic hopes, and in the process live the lives of devils.