GNU glibc has some gratuitous extensions over ANSI C/C99. Unix utilities have long non-standard and non-POSIX based flags.

So what? The OP wrote:

> Those hacks that allow for the ease of use in a GNU userland make programming a massive pain in the ass where nothing "just works" the way it should, because GNU programs and libraries are in flagrant violation of every specification and rule of thumb that has ever really mattered for Unices.

Just because the tools offer additional functionality does not mean they "violate every specification and rule of thumb that has ever really mattered for Unices" (what does that even mean?!). I mean you're welcome to edit files in ed, because that's pure Unix, but I'd rather use a more capable editor.

Posix just states ed, but a pure Unix doesn't have a "default" editor.

But vi/nvi it's useful because you can use commands to replace text inline, or pipe text parts to commands.