OP, I guess this is too late to be of any benefit to you but, we can titre dogs to see if there is still defence against rabies in their system. Research shows the vaccine is effective for years longer than the vaccination interval. I don't know anyone who titres their cat but I'd look into it.
FWIW my Vet does not give the rabies to my 15, 17 and 19 year old cats. Actually they are all closer to one year older so 16, 18 and 20.
Another thought: You know, people can be vaccinated against rabies too. I'd kind of think a Vet who was that concerned might have himself vaccinated. My neighbour who used to work at Connaught Labs making rabies vaccine was vaccinated years ago and last I heard he had himself titred every year to see if he still had resistance. Seven years out and the vaccination he'd had 7 years previous was still producing high enough titre levels. I worked at Connaught too and everybody who worked there, in rabies, had to be vaccinated.