I don't see why we should be serious about using Org for our
documentation, when most people already know texinfo and are quite happy
with it.
I think a reasonable examination of the emacs-devel archives as well as the common practices of most of the people publishing emacs lisp packages today would lead to a very different conclusion. There are several threads about maintenance concerns around makeinfo/texinfo, and many discussions about replacing texinfo with, for example, HTML.There are periodic threads where people claim that they won't try to add their project to GNU because the burden of learning and using texinfo is too high, although those have died down in volume since it became more practical to translate other formats to texinfo.
None of this is to say that Org is the "right" format for Emacs documentation. I just don't think it's correct to say "most people already know texinfo and are quite happy with it".
~Chad