Probably that not everything can be cleanly abstracted as a file.
One might want to, e. G., have fine control over a how a network connection is handled. You can abstract that as a file but it becomes increasingly complicated and can make API design painful.
If everything can be represented as a Foo or as a Bar, then this actually clears up the discussion, allowing the relative merits of each representation to be discussed. If something is a universal paradigm, all the better to compare it to alternatives, because one will likely be settled on (and then mottled with hacks over time; organic abstraction sprawl FTW).
> Probably that not everything can be cleanly abstracted as a file.
I would say almost nothing can be cleanly abstracted as a file. That’s why we got ioctl (https://en.wikipedia.org/wiki/Ioctl), which is a bad API (calls mean “do something with this file descriptor” with only conventions introducing some consistency)