No-one has mention which vaccination this is, but if it is against cat's flu or any of the other vaccinations that are renewed annually, then adult cats, even the old, that have been regularly vaccinated are usually quite safe from infection even if not vaccinated again on the due date.
This does not apply to a rabies vaccination, nor, I believe, to FEP.
If there is a limited supply of vaccine it makes good sense to reserve it for kittens that are only just old enough to be vaccinated. The same would apply to puppies if the distemper vaccine is in short supply.
Quite another thing is, that cats are only at risk of catching any of the infectious feline diseases if they go out, so if any of you keep cats that never do go out, then you don't need to worry.
I would be exasperated too if the vet kept sending reminders after telling me that the vaccine is in short supply!