Ford justified killing Mercury because Mercury wasn't providing a sufficient chunk of market share in the US. Ford recently announced how much market share Lincoln has and it makes Mercury look good.
Justin Berkowitz at Car and Driver made the association, citing Ford's 2010 statement explaining why they were ending the Mercury Brand.
Mercury originally was created as a premium offering to Ford and was an important source of incremental sales. Of Ford Motor Company’s 16 percent market share in the U.S., Mercury accounts for 0.8 percentage points, a level that has been flat or declining for the past several years.
Berkowitz goes on to point out that Lincoln is doing worse than Merc by the exact same metric. Lincoln held just 0.57 percent of the US market in 2012 and it's on the decline, falling from 0.67 in 2011.