> Gašper said that one of the problems is third-party content,
which very likely means third-party buildsystems too. Passing command-line
options through poorly-written buildsystems (or poor use of good ones) is
unreliable.

But here's the thing, if it's a third-party content you really can't tell them you can't use pragma once anyways. It's code that already exists using a feature that already exists.
Making pragma once part of the standard won't change this dynamic.

If you already have it, I assume it must work for you already, of it doesn't, do you actually have something that we should be concerned about?
You can't have your cake and eat it too.