So aim lower, buy a smaller, but reasonable size Publisher.As others have already noted - buying a publisher might net you the book publishing rights for the titles currently in publication. But it may not get you all fo them. And it WILL NOT get you the software, TV, or movie rights - those are separate, and you'd have to get them from each author (if they weren't already optioned out to someone).
Plus, the scales are completely different. Tor Books publishes new titles by the dozens each year. WotC does a handful. It would not just be about having a channel to publish, or content deals they actually wouldn't get - it would become about managing a whole separate business that they don't have the expertise in.