Rumors of the death of the binary distribution trend have been exaggerated.  By "binary" we mean the increasing dominance of the multinational-or-boutique model, with less and less room for the mid-sized firm.*  Speculation about who's next is rampant after the Wilmer-Hale merger, with a focus on the DC market.  It's hard to argue with the logic that to be truly national, a firm needs a presence in the financial capital and in the political capital.

Even if mergers remain "daunting" and if pulling one off successfully is "an extreme long shot," people will continue to talk if approached by a firm that might be a fit.  "It's the responsible thing to do."  And consultants-speaking-anonymously love to add fuel to the fire.

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*What is "mid-sized"? Like SUV's, mid-sized keeps getting bigger. I would posit 150—500 lawyers is about the range these days, with a true "boutique" under 100. Except for firms with a strong regional orientation (cf. Preston-Gates), it's an awkward size.