Snort. Kersnuffle. More nonsense about reviewers.
As far as the proportion of nonfiction reviews goes, I am less concerned about that than the proportion of less-than-happy reviews (since the NYTBR doesn't do vicious ones, except at specific editorial invitationwhich it continues to deny). No matter how good one's evaluation of books based on the cover, whether author's name, cover art, whateverI'm referring to extrinsic properties, as opposed to what is actually on the pagea certain proportion of what one reads is crap, or at least a severe disappointment. For example, I still recall my disappointment of two decades ago when Le Guin's Always Coming Home was published; it is a slightly-better-than-mediocre book, and simply not up to the standards of her preceding (or following, for that matter) work. The difficulty is a combination of reviewers' self-censorship, often refusing to finish a work and review it when it starts disappointingly, inexcusably low standards, and editorial/advertising department pressure for "happy" reviews.
But, if one needs more, and more-considered, nonfiction reviews, one can always go to the New York Review of Books. Outside of marketing-category magazines like Chronicle, Locus, and RT, there isn't very much in national-circulation reviews available to typical readers that emphasizes fiction. Thus, the NYTBR is filling a marketing niche, even with the relatively low proportion of fiction reviews that it now prints; and trying to go outside that niche to an area of greater competition is not the soundest business plan I've ever heard of. One cannot assume that one will maintain current dominance in the new market niche; look what has happened to Toyota in mid- and full-sized pickup trucks, to all of those children's imprints from successful editors that last about three years, and so on.