"
And if this world runs out of lovers, we'll still have each other
They will still have each other, no matter what. And whatever the people around them do, they won't be affected. This just proves that their love for one another is strong. Strong enough for them to say this line. They'll still have one another to love and more. It also proves their dedication since the line says "If this world runs out of lovers, we'll still have each other".