The article almost makes it sound like UTF-8 was a Plan9 invention.

I also don't understand why using UTF-8 was a bad idea. If I start a new OS from scratch, I would have to be brain-damaged to not use UTF-8 as the sole encoding (and other encodings being treated as second class citizens).

I can understand the issue with taking the file abstraction to perfection. If by doing that you are incompatible to most of the software there is, you're going to have a hard time getting developers to develop for your OS. This could be somewhat alleviated with a ioctl compatibility library for the time being.