Christina Lange Your question is a bit old, but I just finished the books and I agree with you. Looking at what short separation is doing to them it doesn't make sense for me that he will just live on. I have tried to read through Deborah Harkness many Q&As, but I can't find an answer. I like to think his wish of growing old with her comes true. At the same time, the books make it clear that no one lives forever. One day they will reunite.
This answer contains spoilers… (view spoiler)[I find it out of character that he'd just let her die. I don't get what's so terrible about changing her. Is being dead not change? What about their children? They're long lived but not immortal either. Is he just gonna watch them all die? (hide spoiler)]
Ian As maudlin as Mathew is he most likely will keep her memory alive. Rebecca, as the daughter of a Weaver/Blood Raged Vampire, will likely live to four hundred years and will keep the old man company for many years to come.