[hobbe]

I'd say they could go for pure sponsorships - just a set of logos with a link to each sponsor's homepage, under the heading "Wikipedia Gold Sponsors". Very few options to sponsor a reduced part of Wikipedia (i.e. sensible to allow on language level, not at keyword level). Probably still would make the foundation a healthy sum of money each year - many large corporations would like to be seen as good-hearted contributors to the world's largest source of free curated information.

[varaon]

Even if they adopted an official policy of allowing criticism, there would still be a stigma to including criticism in articles about the sponsors.