Miki, Kelvin, Koji, and I think James G2 can be purchased separately.  Miki gets modest support.  P5 figures were never well supported and most Poser users have those figures (they're in P5-P7).

It's more a phenomenon of where people are buying Poser content than anything. When the first Victoria and Michael came out, they were far better figures than the P4 characters before them. The new EF Poser figures aren't significantly better, or (in my opinion) even as good as the current DAZ figures. Apollo Maximus was a better figure, but didn't grab enough of an audience. Unfortunately, Anton pulled that figure from the market and that kills any chance of that figure ever having an increased user base (and probably hurts Anton's efforts at releasing a new figure in the future).

It doesn't mean that a new figure won't eventually wrest DAZ's grip on human characters in the future, but it will take something that's much better than what is available today.