The more time I've spent around cats over the years, the more I've come to believe that a large (probably the largest) component of how long they will live and what health problems they will have is genetic.
That said, I also think that the choices we make for them can either create an increasing downward spiral with their genetic tendencies toward disease and shortened lifespan, or fight against those tendencies to give them the best quality of life that would be possible for them.
A good diet isn't a miracle cure. It's not a magic bullet. Cats are living creatures, not machines, and there's no absolute formula to keep a living creature free of any health problems.
However, I believe in doing the best we can to keep our pets healthy, even while being realistic that it won't solve every problem.
The Boys: Maisie, Zephyr, and Cullen.