That's a bit of a myth that doesn't hold water (pun intended) anymore.
Many cats do just fine on dry food only, including mine. You do have to keep an eye out for urinary tract infections and need to ensure a "appealing" source of water is always available - i.e. not just a bowl of water as cats have a suppressed thirst response unlike dogs who drink voluntary. My cats have never suffered any complications from dry food only - quite the opposite for instance in terms of the health of thier teeth especially.