*when DAZ releases a MAJOR new female figure
that CANNOT be used in poser at all

Um, could this be Vicky 4? Once Studio is up and running (and offered for free), why should DAZ support Poser any more?
Personally, I think this is a very real possibility and I would also be willing to place bets that DAZ takes this approach with either V4 or Mike 4.*

I don't think they'll do that. I suspect they will make two versions, one rigged like V3 for Poser users, one with D|S-compatible advanced rigging for D|S users.

Meanwhile, Poser 7 will have its own advanced rigging system. It will be backward compatible, so you can use the old figures, but all the cool stuff will work only with their own figures (Fei-Fei, anyone?).

People will be forced to choose, or be locked into the 1990's technology of Poser 4.