This is just my opinion and my experience with the cats I've owned. My previous cats received their initial vaccinations, but I did not do annual boosters - maybe every 3 years. Rosa and Regan have NEVER been fully vaccinated. They both caught Calici virus in the rescue center they were at before I adopted them. They started showing symptoms after the first of their initial course of vaccinations. Of course them catching the virus invalidated that initial vaccination as Calici is part of the standard vaccinations in the UK and meant the course couldn't be completed until they were better. I have my suspicions that they might have got a batch of vaccine where the Calici portion wasn't fully dead, but of course I can't prove that - no other cats in the center had the virus either before or after and the timing was a little suspect to say the least.
My sister-in-law had at the time just lost a cat to heart failure. He had also had Calici a number of years earlier but she had had his vaccinations boosted annually. Every time he was vaccinated, he got a flare up of the virus and was sick for a couple of weeks. In the end, that was what caused his heart failure - the repeated strain on his system became too much. Given that, by the time Rosa and Regan were well enough to be vaccinated, I'd already almost lost Regan at just 4 months old to pneumonia as a secondary infection, I was of course not willing to risk their health any more than I had to. They've always been indoor/closely supervised outdoor cats and neither has ever caught anything from either other cats in the neighborhood, the other cats in the house since I moved here or from any vet office. They were rabies vaccinated before I moved here as my UK vet insisted it was legally required (I later found out that it wasn't, but by then they'd already had the vaccination as I had to wait 3 weeks for blood tests to be done to make sure it had been successful). But other than that, they've had no other vaccinations at all. My UK vet was never entirely happy with that decision, but accepted it was my choice. When I explained it to our new vet here, they just noted their files so that I don't get annual reminders but I haven't been put under any pressure at all to vaccinate them.
When I spoke to the boarding vet where Rosa will need to go any time we're away on a trip, they said they wouldn't recommend vaccinating her because of the diabetes and I will just need to sign a vaccine waiver each time I check her in for boarding (we already do this for our CKD kitty, Shadow, who no vet will recommend for vaccination).
I would never suggest that vaccinating a cat is necessarily a bad idea, but I do know it's possible to keep them healthy without if it's something you're not happy about doing.
And btw I totally agree on the insurance - I had a UK policy for my 2 but couldn't transfer it here as it was a UK only company. Of course by the time I got here they were 10 years old and couldn't be insured at all with most companies, and the premiums were outrageous with the one or two that would have taken them. Now I wish I'd paid the ridiculous premiums with the amount Rosa's treatment has cost, but now there's no way I could get insurance for her diabetes anyway so the time for reviewing that is over. But yes, she's cost more in just a couple of months than I've spent on my own medical in a year and a half!
Plus, should we ever need to take Hannah to Cornell for treatment, they won't even let her in the door without a current rabies vaccine. 
