Personally, what I would do now (and what I do now for my 4 cats, 2 of which are elderly) is put money into a bank account that is untouchable. Put in the same amount as you would pay for insurance.
If your cats are young, then the chances of them having an illness that would cost more long term to treat than you would pay for cat insurance is very very small. Nature is a lot harsher on cats than on humans, and weaker cats just don't make it through kittenhood.
ALso, young cats (under 2) are the most likely to get run over etc, so insurance is again more likely to be a waste of time than with an older, more sedate cat.
I spent 6 years paying for cat insurance, I can't afford it any more, but it doesn't massively worry me - one of our elderly cats had to go to the vets last week, and the cost of the checkup and treatment was only £10 more than the excess I would have had to have paid if he was insured.
Obviously, being uninsured is always a risk, but pet insurance (cat insurance especially) is one of the most lucrative schemes for the insurers.