This has already been discussed many times. You only have to see Please make a new software for Haiku, not only a port to know what I mean.
I prefer to focus on a native shell, which is integrated with the API.
If we need POSIX support, we already have Bash, and whoever wants to use it, can use it.
If we really want to move forward, and differentiate ourselves from other operating systems (for example: any generic Linux distribution (or any other UNIX/POSIX compatible)), we have to think about projects like this.