100% sure? Not even the smallest doubt? I don't know how old you are, but can you remember every rhyme or song you were taught in your childhood? There's not a single one you've forgotten, or even forgotten a single line to?

Also, if you teach a kid a rhyme, can you trust he or she will never repeat that rhyme in public or the presence of someone else?

Too high risk for my liking. High durability physical storage, such as metal, would be my preference for a 20 year hold.

It won't. The rate of increase in supply will decrease. The supply itself will continue to increase, albeit more slowly.