After the fall, God could have made Adam and Eve sterile and allow the human race to die out. He could have created a new Adam and Eve that only said yes to God and his love.
Could this have something to do with Isaiah 55:11?
11 so shall my word be that goes out from my mouth;
it shall not return to me empty,
but it shall accomplish that which I purpose,
and succeed in the thing for which I sent it.