There's still too much old content being used for people to give up P4 or PP or even P5.

But you can use P4 and PP content in P5 and P6. You sometimes have to adjust the textures a little, but backwards compatibility is pretty good. The existing content shouldn't stop anyone from upgrading. I use tons of P4 and PP models, but haven't actually used P4 or PP for a long time.

I think CL will be releasing "Poser Artist 2" soon. (IOW, Poser 5 in a new box.)