The reason I said Mike was not clear was he claimed each edition sold more than the one before it yet 3rd ed is referred to the silver age of D&D with around 1981-1983 being the golden age. He did also say initial sales do not matter to much as modern D&D it seems is heavily frontloaded to the 1st few months, TSR era was a slow burner as such with smaller print runs sold over a longer time frame.
He isn't the only one saying each edition has outsold the previous one. Dancey says that up to and including 4E, where 4E outsold 3E's initial sales based on pre-orders alone.
It isn't that initial sales don't matter. Of course they do, because sales matter and you always want strong sales, market penetration, and excitement around a launch. But, the key is that the interest remain as high as possible. And, ideally, across the line. One of 1E and 2E's big problems was that they had no idea how to correctly sell product beyond the core books. They had a poor understanding of what customers want and often were ignorant regarding the profit (or lack of it) behind each product. (Dancey writes plenty on these subjects, but others have said the same before).
This is why we can think of the '80s as the golden age (because D&D was a massive brand of which most people in the US and many countries were aware) all while it was terrible from a business perspective (TSR was overhiring, overspending, underselling, etc.). Like many businesses, the highest years of revenue concealed the biggest problems.
What Mike Mearls is saying is that the initial sales are terrific, but it is smart to be cautious and see how the game is received as time passes and more products are added. This isn't just the core books, but also the adventures, the licensed video games, the licensed miniatures, the licensed board games, etc. Fast-forward to March, when the next storyline theme is released, the next officially licensed adventures come out, and so on. Is D&D even bigger then? The same? Smaller? If the licensed products continue to well received, as they have been, then this could pave the way for much more. If not, you have to rethink the model, retreat in some areas, and try new things. A lot can change in a year.
None of that takes away from the incredible start this RPG has had. 5E's initial numbers are nearing the point where they dwarf anything before it. If somehow D&D can regain some of the '80s spotlight (so far so good), it could bring in a new golden era. It doesn't even have to outshine the '80s to be great for RPGs. And, preferably, any golden era comes with much higher profitability and business sense. This seems to be in good hands so far (just look at how licensing is being used to handle traditionally unprofitable products such as DM screens and minis).
