That's not true.
1. We don't know how many players there are. Neither do they. Nor do we know how many people have ever played D&D, nor do they.
You know that there are these things called "polls" and "surveys", right? You call up a certain number of people, ask them some questions, use some of the answers to those questions to check how well they sync up to the population at large, and then extrapolate the results of the other questions to get proper answers. It's not a perfect science, but it's pretty damn good.
2. There's nowhere near more content than ever. WOTC produces almost no content now. 3rd edition had how many books? How many issues of Dungeon and Dragon existed? How many 3rd party 3.x products existed? 5th edition, at its current pace, will need decades just to catch up to 3rd edition.
Here's the thing: that content still exists. Most of it is up on DM's Guild. And it's all D&D content - some of it might require some massaging to mesh with 5e, but generally not that much.
3. In 1980, or even 1990, how many book stores existed? How many exist now? How many magazine stores existed? How many exist now? Not even touching how many hobby shops existed then compared to now.
And these days there might be fewer hobby shops, but many of those that do exist run Adventurer's League stuff and such which brings in more players. Many of those then go on to play outside the shop with their own friends as well.
Marketing D&D?? They haven't marketed it at all. They're really not even producing D&D. Most of the product for it is made by someone else, and what little product they have released is largely just content from the 1980's with a new coat of paint. Heck, they're so hellbent on not producing D&D they turned down a few proposals for video games.
The last major traditional product (e.g. not spell cards or such) that wasn't produced in-house was Sword Coast Adventurer's Guide. Since then we've had Curse of Strahd, Storm King's Thunder, Volo's Guide to Monsters, Tales from the Yawning Portal, Tomb of Annihilation, Xanathar's Guide to Everything, and soon Mordenkainen's Tome of Foes.
And not marketing? What do you think Dice, Camera, Action is? Adventurer's League? The videos about D&D stuff that D&D Beyond puts up with snippets of designer (or other) interviews? The ARG they're doing leading up to the Stream of Many Eyes?
Now, there are certainly things I wish the D&D team was doing because they would cater more to
my tastes, but the stuff they're doing is definitely working out well for them.