From what I'm gathering, you only need to disprove it's false if it's going to have some form of application or another, just in case there is a single situation out there it won't be true. This is a process that is likely used just to be on the safe side.

IN short, if you can't disprove something to be false, it hardly means it isn't true. It just means the community wont' risk using on the off chance it's wrong.

Given the impractical use of .999... and the fact I see no way it could ever come up, having to disprove its false is an entirely pointless excersize, regardless of if it was possible or not.

Either that, or the Idea of disproving falseness has nothing to do with eliminating risks, and all mathemticians are just Elitists assholes. And if thats the case, it still has no real relevance to this conversation.