We have the numbers from last year from a WotC talk. D&D in total sold each 29 million in the first and 2nd year of 5E. Which is absolutely god awfull, because 3.5 sold 25 at the end of its life and we know that 3, 3.5 and 4 had way stronger sales in the beginning.
Mike Mearls LITERALLY had to compare the 5e sales with the only year where not a single new D&D product was released to make the numbers look good "we doubled the numbers" yes compared to a year with nothing.
Also what does sound better? "We have to delay DMG because its not ready" or "omg we sold so much more than we estimated that we have to print the other book later"?
Here is the post linked in which the yearly numbers of the sales were given
D&D 4E - WotC, DDI, 4E, and Hasbro: Some History
We have the numbers we dont need to believe marketing anymore. Even in the linked interview you can hear that in 2013 WotC was close to losing D&D, so of course they make their Marketing in 2014 sound like 5E is a huge success.
Also Mearls of course want to make it look like his work did well, he is profiting from people thinking that 5E did well from the beginning, that sounds better than "the lead designer of the 2 least successfull half versions of D&D"