Two thousand or so years ago, Rome was a democracy. Then it changed to an empire. Doesn't that show that democracy is unsustainable?
Ten thousand years ago (or so), before the dawn of civilization, societies lived in libertarianism (is that true?)
Libertarianism is not anarchy. Anarchy is what we had had prior to civilization, not libertarianism.
Under libertarianism, there are still "countries, nations, governments, regulations, etc.". It's just that there are fewer laws and regulations because libertarianism generally requires a higher bar than majority approval and is more restrictive of responses.
A lot of the problem with Roman democracy was that the state maintained large standing armies. This empowered generals (like Julius Caesar) to conquer Rome. A second problem is that many Romans found the society of the time rather decadent. Much of society was addicted to government spending on bread and circuses. Many Romans welcomed Caesar.
Note that a more libertarian Rome would have spent no government money on standing armies, bread, or circuses. Of course, it also would have been a lot smaller as well. And it might have been an easier target for other armies, although some would dispute that.
No form of society is inherently stable. Dictatorships tend to devolve into monarchies. Monarchies tend to devolve into democracies. Democracies devolve into dictatorships. Libertarianism is meant to knock off many of the rough edges of democracy. But it requires work to maintain. And people have to accept that not every result is going to be fair and good. Fixing those bad or unfair results moves us further towards tyranny, as the forces against fairness and goodness have as much influence as those for it.
Look at the gay wedding cake issue. Note that both sides feel that their position is right and equitable. But the two sides come to opposed results, even though they agree on many of the premises and data. Regardless of how it's handled, some group of people are going to be frustrated.
Governments are run by people. It's difficult for them to be more intelligent than the people that they serve. If anything, the tendency towards democratic compromise makes them dumber. A lot of libertarianism is about limiting the ability of the government to impose majority consensus on minorities. That's going to be resisted by those whose goal is to impose the majority consensus on some minority. I have the might therefore I have the right is a natural way of thinking. We've been doing it for tens of thousands of years.