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.
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.
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.
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 saddest part is, if they actually tried producing D&D they'd probably be able to grow their market to huge proportions.
You are thinking about DnD 5e and not about DnD in general.
- They have been producing a reasonable amount of DnD 5e official books.
- They have produced a very big amount of PDF support for 5e through DM Guild.
- There is a ton of unofficial DnD support in DM Guild (and several great authors they are showcasing).
- They have put nearly all past DnD content available in PDF (and more in print on demand every week) on DnD Classics.
- They have started supporting digital tabletop tools.
- You are right that there is less 5e 3rd party support (from big companies) than during the 3e era. Still, the 3rd party support 5e has gotten is of amazing quality (Adventures in Middle Earth, all Kobold Press stuff, Goodman Games...).
And that's just books for the RPG. Then you have miniatures: most people I have met in the AL have bought at least a few of the Nolzur's minis, with GMs having a considerable amount of them. And DnD themed boardgames, which have had a pretty good reception. Heck, they have even released DnD PDFs to tie in with Magic the Gathering, something no one would have imagined in any previous edition.
Another poster said much better already: they want to grow the DnD brand (that's why they don't want to split into half a dozen of settings). IMHO their strategy is a sound success.
So at least for me: kudos to WotC for managing the brand in such a great way, and for proving so much DnD to its fans.