I think I'll have to go with the slight evidence from Gygax/ TSR/ WOTC (see above) over your countering opinion. Unless you have any better data.
What "countering opinion"? In what you quoted, I am stating something that is essentially fact. It is nothing more than an inevitable conclusion drawn from what has been said in the two relevant threads. See for yourself:
1) The only people who have real access to the relevant data for both 1E sales and 4e sales are the people at WotC.
2) The people at WotC have said that the sales data for 1E is too imprecise to make any such comparison.
3) Thus, it is impossible for anyone to make any kind of comparison between 1E sales and 4E sales.
4) Any statement to the effect of "D&D was at its height in the 1E period" requires a comparison of sales data across editions.
5) Given 3 and 4, it is impossible to
prove the claim that "D&D was at its height in the 1E period".
As long as we are talking about objective, non-anecdotal data about the popularity of D&D (which essentially means detailed sales data and market research), which I consider to be the only measure that can be used to
prove this kind of claim, then my argument is valid.
Anyways, aren't you being a bit deceptive when you say that "slight evidence from WotC" is proving your claim? An employee of WotC has gone on record to say that D&D is as big or bigger now than it has ever been. If you want to dispute that claim, you need to find a stronger basis for disputing it than a comparisons between modern player estimates and some old number made by a man who did not necessarily have enough information to make the claim he did. We don't even know how the two different numbers were calculated. I don't hold that kind of evidence to be sufficient to prove your claim at all.
Anyways, I am still sticking to the conclusion I made based on my MtG analogy: D&D was much more visible in the 80s because of its novelty, but that probably did not correlate to either sales, sustained popularity, or anything else of the sort.
However, I am curious if anyone actually knows anything about the relevant sales data for Magic the Gathering, to make my analogy more complete. Without that, I admit it is a bit flawed...