Do we have any book sales charts from June-August 2008 to compare? Can we see how well 5e is doing next to 4e?
I don't know, but that's not what I mean in terms of a success. I'm going just on memory, but as far as I can remember, 5E is FAR more successful as far as the community reaction goes. I assume that you remember edition wars of 2008-10? It was rather fiesty. I know it is anecdotal, but I find it telling that we keep seeing old EN World members come back and say, "I haven't played since 3E but like 5E" or "I left the boards in 2009 because this place just got nasty."
Now when 3E came out people were, for the most part, ecstatic. The last few years of D&D in the late 90s had been rather weak and the game was a mechanical dinosaur compared to the innovations that had occured in the previous 10-15 years of RPG design. 3E was D&D coming into the modern era, and enlargened the community. There were some who didn't like it, but they were a small minority. The voices who didn't like 4E were plentiful and loud. While it is early, I don't envision anything similar to that.
But the question, I think, is more around Mearls strategy of focusing on the brand name and whether that can bring about a new golden era. I think the first challenge is, for the most part, a big success: selling the game to existing AND lapsed players. We shoudn't understate the importance of that. At the least it means that D&D 5E should have a strong run, certainly better than the three and half years of 4E, even if the Great Plan doesn't work out.
So as someone said, while may not see another Golden Age (1E, late 70s-mid 80s), we may see another Silver Age (3.x, first half of 00s) or at least another Bronze Age (early 2E, 1989-95ish). What we are unlikely to see is another Dark Age (4E).
Just to be clear - I am NOT criticizing 4E as a game. I enjoyed it for a few years, although had some of the usual issues with it. I'm calling it a "Dark Age" for how it was received by, and influenced, the community as a whole.