Gettext is a SunOS invention and other implementations are expected to follow
the definition from the reference implementation.
That implementation was the starting point but I didn't just copy it. We (mostly François, Peter, and I) fixed many shortcomings to make the API actually usable. Without that additional functionality the already-standardized message catalog mechanism would certainly have won.
Do you really like to require SunOS to loose backwads incompatiblity?
Overly dramatic. You just need one mode that is POSIX compatible. Many GNU tools use POSIXLY_CORRECT_