"eventually charge for the Pro/Advanced versions of Studio, as was originally envisaged..."

They tried that for a little while and it was counter productive. I seriously doubt they will ever go back to charging for the product when they generate far more on content sales. Any time you have an "entrance fee" you run the risk of limiting your market. Additionally, the vast majority of users are on the same page version wise and that makes the program easier to support.

www.Calida3d.com
Daz studio and Poser content creators