If you think the nation state is transient, you probably watched a bit too much Star Trek. The borders are certainly very flexible but the association of groups of people battling against others has been going on since humans appeared on earth and in my view will continue until we're extinct unless you can somehow eliminate ambition, greed, jealousy and a few other fabulous human traits.
Since you love to nitpick, more than half of HK was a part of China during the British lease and it made sense that it reverts to what China is today. In any case, they wouldn't have had it any other way so there wasn't much of a choice.
Britain did well in ensuring that there wouldn't be any drastic changes in HK's way of life and left far more gracefully then it did in India...