Well, think of it this way.
A housing development has a rash (pun intended) of break-ins.
They get together and decide to institute mandatory installation of alarm systems.
The number of break-ins goes down in direct proportion to the number of houses have alarm systems installed, until all the houses have them installed, and the number of break-ins is almost, but not quite, zero per year.
After a while, people start to think 'we don't have a break-in problem, why are we mandating these alarm systems?'
New houses under construction start to be built without alarm systems. What do you suppose happens to the break-in rate?
The price of freedom (from preventable disease) is eternal vigilance (of vaccination rates.)
It's real easy to say 'we don't need vaccines' when you've never seen a playmate in polio braces, or when pictures of a wall full of children in iron lungs is a quaint historical anachronism. When you don't have an Uncle Bob who's sterile from a bout of mumps. When having a dead sibling is unusual, and probably the result of accident or something, and not 'measles.'