I don't really understand why the relative number of D&Ders (as opposed to the absolute) matters.
When asking is the current work that is costing more to make these days worth the same as the past you have to look at your bottom line.
That bottom line includes the percentage of people playing. Now you can claim superiority of % of RPG gamers that buy D&D v other games, is one way to look at it, but actual popularity of the game itself must be based on all consumers.
So if 10% of the people played D&D in the 80's and only 3% of all the consumers that exist now play D&D, then something could correlate.
You have to look at that and see if it is your product, or the people.
Is your new product not doing as well because of problems with the product being something that consumes dont want.
You cannot make a blanket statement that more copies have sold this decade of a new game compared to a past decade, and leave out the fact the population has grown 10-fold in that time.
It is lying to people. What marketing departments and firms are made for.
But the accountants can tell the truth by checking past ledgers combined with the censuses taken of people in the country or world.
If you only want to know if you are selling more copies, then you can be blinded by the other information and look at the data with blinders on.
If you want to know how well your new product is doing compared to an old version of it (see Coke Classic/New Coke) then you must look at the entire consumer base to tell if the percentage of people is equal to a state in the past to see if you are doing as well as then.
I think it is NOT doing as well as it was then. That is also a job of marketing, but a hidden part not shown to the public as the public is only offered the advertising section of the marketing department in order to sale more product. This is for all companies.
So why should the consumer or gamers care? They don't have to, but some do.
So it is interesting for them to look at all the numbers to compare the "hype" vs reality.