What of fundamental value was lost when Alice died that Betty's birth did not replace?

Hard to say. Notice that in such examples we are past the point where the value of things is motivation by instrumental value (i.e. such thought experiments try to strip away the component of value that originates as instrumental value), and terminal value is not expected to be easy to enunciate. As a result, the difficulty with explaining terminal value is only weak evidence for absence of said terminal value. In other words, if you can't explain what exactly is valuable in such situations, that doesn't strongly indicate that there is nothing valuable there. One of the few things remaining in such cases is to look directly at emotional urges and resolve contradictions in their recommendations in terms of instrumental value (consequentialism and game theory).