The empirical evidence that we have available seems to indicate that anarchy is viable if people are either accustomed to it, or otherwise actively want it. This prevents a power vacuum, because people aren't seeking a ruler to guide them. In situations involving governmental collapse or some other rupturing of the social order, people are expecting that guidance and not receiving it. This allows a new, usually more violent authority to step in and take control. Obviously we want to avoid that.

Note that the examples we have of semi-anarchistic societies aren't perfect examples of what we want. Some only lasted a short time, or were small scale, or had some other flaws. They do serve to illustrate parts of anarchist theory, though. There are also various projects that do the same. The internet is one major example; a global information network used by 2/3rds of the world's population, but without a world government to create and manage it.

We have yet to see a large-scale, long-lived attempt to fully apply modern anarchism. At least part of the reason for this is that the left got intellectually derailed by Marxism and its derivatives for about a century. Prior to that, anarchism had been rising in popularity. We've been growing again for the last few decades, so we'll see what the future holds.