The Teflon will wear in preference to the balls. The Teflon is easier to replace than the balls.
And face it - they won't last as long as the original metal cups - but they're SO cheap and easy to replace. Buy three sets and you're covered for the rest of YOUR life.
Being a good custodian means leaving it so that those who go beyond you will have it easy to keep it up, and be more likely to do so.
Joe K