That was Amit's doing, actually. Around 2012, with the success of Knowledge Panels and the acquisition of Metaweb (FreeBase), he decreed that search would become a "conversational answer engine", devoted toward getting you the answer to your question as quickly as possible. The Hummingbird ranking update was part of that - it made it much more effective to phrase your query as a question rather than a bunch of keywords. Also renewed integration of Google Now into search, expanded Knowledge Panels, cards on mobile search, bigger investments in voice search, etc.

Which makes me wonder what search network ads look like in a hypothetical world in not so distant future if audio interfaces were primary search tool and output for the results.

Personally, at this point I'd rather pay Google a monthly subscription and get everything ad-free. That would align their interests with their users who would then be the customers vs the product and free up Google to do what is best for them in cases where ads may not make sense.

I wonder how many real search engine queries only need to return a Wikipedia page.

Indeed. Information concentrates, just like capital.