UNIX grew to the complexity it has because it was meant to be used. Plan 9 is not used because it was not allowed to acquire useful constructs.
This parting ways with "the UNIX way" was necessary and complete in order for the system to become what it is today. Some things could be improved and made more proper and UNIXy, but for the most part it's an incredibly useful and functional system. The OP article is spot-on with this realization.
In other words, the ideal UNIX (for example, Plan 9) can't compete in the real world, because the concepts behind it (the UNIX way) don't actually compete in the real world. It's based on a set of very interesting, very elegant, and mostly false precepts.
Edit: Also, Plan 9 has HTTP servers, CIFS support, central authentication (factotum), etc. I suggest you Google them.
I know plan 9 has factotum, but "google it" sure doesn't help me find a way to use it to authenticate windows clients. Or authenticate anything other than plan 9 for that matter. Aquarela doesn't even claim to work, and it is actually much worse in practice than it claims to be. And it can't be part of a domain can it?
I'm not trying to be critical here, but when I ask "what can I actually do with plan 9?" I am genuinely interested in a serious answer. Stuff like "well there's an unsupported kinda buggy thing that doesn't actually do what you need anyways" is just a less honest way of saying "you can't use it for anything".
You seem to seem to be looking for "an alternative to Unix." If that is the case, Plan 9 probably isn't what you're looking for. It is, however, useful in ways that Unix is not. It probably won't replace your specific Unix needs, though it largely replaces mine. Hope that clears that up.
Since plan 9 largely replaces your unixy needs, could you share with me what those needs are, so I can try to get a better idea of what I might be able to use it for. I am not looking for an alternative to unix, I am looking for any practical way I can put plan 9 to use, rather than just tinkering with it on an old spare machine for fun.
I did not interpret your post as a criticism. Why do you think I did?
> I would like to know how I can actually use it for something
You can use it as a source of ideas for implementing operating system and network services. That may not be useful for you, but everyone is not like you. Others have found it useful to use directly(coraid).
If you "know about it", then you would also now that you cannot use it for something like the things you mention. I cannot see how your questions are compatible with "knowing about it".
Even its developers doesn't use it now --although they have ported their favorite userland tools from it to other systems.