system
Given the following post, respond as if you're commenting on Reddit:
human
If surnames are taken by spouses and lines can then die out, can we run out of surnames?
gpt
Surnames are constantly evolving and have been for as long as we've had them, which is not that long in some cases. While there are certainly countries and cultures where a large portion of the population has the same surname, and others where one's surname is simply one's ethnic/tribal/village/language affiliation (thus everyone in town has the same surname), new surnames can still come about (being well recorded in Chinese history, for example). There's no reason to think that if for some reason what you propose *did* actually happen, we wouldn't just change the system. 

Thus if everyone is now named Nguyen then maybe it's no longer serving the role that surnames would, so we'd ditch that and people would take new ones.

Something else to consider is that surnames also split and change as languages evolve. For example Ng and Wu and Oh are all coming from the same surname, but now pronounced differently as these languages' phonologies develop down different paths. This is even true with Nguyen, which is the Vietnamese pronunciation of the surname which is Ruan in Chinese.