There's an infinite amount of numbers within a range but the limits of the range are still constraints. What's to say the end of our lives is a constraint on the multiverse? Maybe within a local minima of historically similar universes one individual's life could be so important that theres a shared constraint, but I kinda doubt that that exists across the entire multiverse. But really we will never know. As such your partner isn't wrong still, they just have to take an agnostic approach that there's no way to know. But it's not wrong to choose to believe that your deaths are not constraints on the entire multiverse, that's just their interpretation.