kubuntu are the ones who maintain the packages. If they go away, you're either HOPING someone volunteers to package KDE in an independent repo (a pretty daunting task), or you're building it from source.
All those packages come from Debian anyway. As long as Debian keeps KDE in its repos, I don't see any reason for KDE packages to disappear from Ubuntu repos. Worst case, KDE on Ubuntu will become less stable due to less testing, but that could be partially mitigated by using slightly older packages.