Basically what Mirrorball said just below your post here. If WotC only sells 3 products a year, and two years go by that I don't buy anything, that strengthens interest how?
Basically, I don't buy that WotC's designers are working on plushies, or video games, or board games. That's nonsensical. Working on ancillary products to drive interest to a product line that doesn't exist? What is a Waterdeep board game supposed to drive interest in, if there's no Forgotten Realms? It lacks any context. Also, I'm pretty sure that the game designers are game designers. Not programmers.
There are obvious spaces to be filled in the existing core. I've mentioned them. I think we'll see them. But the amount of support 5e has received since the DMG came out has been...what? 12 pages of Eberron material and a Sage Advice article that doesn't answer any questions? That concerns me. There's no anticipation that I've noticed for the Princes of the Apocalypse adventure, and nothing else to be interested in. It's one thing to expect declining sales after the core 3; quite another to write off the sales in advance.
Here's the hard truth: Wizards of the Coast doesn't expect you to buy everything. But they're also not making products just for you.
This is the catch. For the last two editions, all the D&D eggs have been in a single basket. All D&D products were really focused on a single audience: the RPG players. So the game lived or died based on how that one audience wanted to spend their money. And if that audience felt stretched thin, then sales would drop. If that one audience felt they had enough product, then sales would drop. If another game or product came out that prioritized that audience's funds, then sales would drop.
Now the D&D is being spread out. WotC is focusing on spreading out to to board gamers, video gamers, miniature war gamers, comic fans, and yes the RPG fans. So they can pull in the same revenue as monthly books but target different audiences, potentially increasing total profits.
And WotC can do so while licencing out products, keeping their own staffing and overhead costs low. Because WotC is all about maximizing profits.
This does mean fewer RPG products, but it should mean its significantly easier to afford said products as there's less cutting into the ol' wallet. And the rarer products are more exciting, since they're less common and everyday. Which might translate to higher and more sustained sales.
It's a little like going from a spoiled only child to having a couple siblings. Suddenly you have to share and not everything can be about you.
And, hopefully, with fewer books they won't need to reboot or relaunch the game after 2-4 years.