I wonder how successful Wikipedia would have been if it weren't for being the first result in almost every Google query.
For reasons related to the above, afaik Google has never tried to nerf its prominence in search results, unlike profit-seeking content farms. That's probably an underestimated reason for its SEO success as well.
On these assumptions I don't see any reason why Google should "nerf" its results, since they effectively present what the user is most likely to be looking for :)
And on a related note, "what the user is most likely to be looking for" is path dependent. Unlike other content sites, I rarely bother using Wikipedia search, because I can invariably find the page by Googling from my browser. If the relevant Wikipedia result wasn't consistently on the first page of Google, I'd tend to go direct to WP when actively looking for Wikipedia articles like I do with other less search-optimised but often superior websites. And if that was the case, Wikipedia would almost never be the result I wanted when Googling subject matter, much like although I'm often interested in BBC News, finding the Beeb's take on a subject is never the reason for me using Google News.
With other sites, you often have to wade through ads and read the whole thing just to answer a simple question. And most of the time, I'm just looking for details on things I already know.