Indeed. However, even if a pure OS4 port appeared that made maximal use of its Interfaces mechanism (and new Interfaces also), this doesn't preclude parallel GTK ports to either AROS or MOS and in fact would likely encourage developers to get a move on with it.
Regarding this issue, I much prefer the notion of portable APIs that are implemented in the most optimal way for any specific platform than "totally generic" code that makes minimal use of the intersection between several of them. But regardless of which route it takes, someone needs to make the first move.