I think they want to make sure they define what D&D is in the marketplace since they intend on leveraging the brand effectively. It isn't that WotC doesn't want 3PP support, they just want to control it. And if the big 3PP companies they are courting go along, they will.I'm not as sure about the commercial implications for WotC. I'm not denying that there are some sales made by them that are driven/supported by 3PP materials, but I have doubts about how big this is as a proportion of the overall recent growth of D&D.
If Hasbro wants to "win back their IP rights" is this really directed at Paizo's present position? Or any other 3PP? Or at controlling further growth that they envisage flowing from the anticipated success of the movie? To me, it makes more sense as a future-oriented concern than vindictively trying to destroy the present ecosystem.