Mesh mum, well you're never going to prove a negative are you.
That's a bit like saying. This is my child who didn't die in a car crash because we bought a Volvo/Saab instead of a reliant robin.
But that's the problem: none of us knows if we still have our DC because of vaccination, or improved sanitation, or any of those things. Therefore it's possible for people to, frankly, delude themselves, and decide their baby doesn't need the vaccine against polio because nobody gets it now. Or that a rubella vacc is pointless.
As Side says, you can't prove a negative. But when deciding whether or not to vacc a baby, it's an issue it might be wise to think about. The crushability of certain makes of car is why parents decide now is the time to get a Volvo. So why not vaccinate the children too?