The Locked in the Basement Version of Universalism

I think universalism should not be on the table for Christians anyway. As you mentioned, it underestimates the hardening of the human heart. It also doesn't take into account the common grace that we're experiencing here on earth right now. The atheist knows what love is because God exists and gives that kind of common grace to everybody else. They're experiencing God's love and goodness in his creation whether they realize it or not.