I think there's a very good argument for not vaccinating adult cats.
But if they're insured you may find that not having them vaccinated will cause problems with certain claims. Alf had calicivirus despite being regularly vaccinated. It's one of the things they vaccinate against. Had he not had proof of regular vaccinations none of his claims for calicivirus related health issues (around £2000) would have been paid. You may say, well clearly the vaccinations didn't work and you'd be right. Somehow he still got it and it was a problem but he was still insured. Much as I adored that cat he was a bloody duffer in the health stakes.
Also some catteries won't take cats without a history of injections. And if there's a sniff of calicivirus the cat won't be taken either.
Fortunately I've no intention of ever putting Harvey in a cattery but I doubt they'd want him as he is suspected of having calicivirus but I've never had him tested.
Vaccinations are a racket. Lots of doggie people will only give them their first jabs, then never again. If we didn't go to visit the Coglet, so have to board The Princess, I wouldn't get her vaccinated again.
There's a lot of evidence to support that after their first vaccinations, (dogs) they have enough immunity to last them the rest of their lives. Vaccinations are known as the vet's pension plan for a very good reason.
I won't use chemical wormers or tick and flea treatments either, far too toxic. I use human grade diatomaceous earth for both.