I don't think that's necessarily true. I think WotC is counting on sales being carried by attracting more people to the hobby through licenced products.
Actually, I'm not convinced WotC are particularly bothered about sales of RPG books at all. Sure, they'd rather have them than not, but I suspect they're more interested in getting people playing. Because if they can say "we have 8 million regular players", that's a much more attractive licensing proposition than "we have 1 million regular players".
Even if D&D the TTRPG sells really well, it's still becoming more and more niche. In the 90s, I'd say the majority of RPG gamers were playing a D&D product, be it the TTRPG or the excellent CRPG offerings, or whatever.
Today, only a tiny fraction of those who enjoy RPGs are playing D&D, because the market has exploded in other areas that WoTC did not have a good presence in.
Ah, but there's the rub - D&D is only more niche in percentage terms, and that only because the potential market is now so much bigger.
A few years ago, WotC did a presentation at which they gave a figure for their current player base (which was either 1 or 6 million), and at which they said there were ~20 million people who had
ever played. D&D has
always been tiny, even at the heights of the fad in the 80s.
The difference being that back then D&D was a major player in a tiny "geek culture", whereas now it's a tiny player in a "geek culture" that has somehow become
huge, what with the success of the LotR films, the endless superhero films, "Big Bang Theory", WoW having 7 million monthly subscribers (down from 10m recently), and so on.
D&D isn't becoming more niche. It's staying the same size. It's just that the niche has become a huge gaping chasm.