As I pointed out in our latest episode of Inappropriate Characters, I think that the sign of it is in the Candlekeep book. It's got every look of being absolute woke garbage, almost every adventure written by people with no real background in RPGs, full of woke signaling.
Candlekeep seems be a litmus test for them pushing the envelope beyond the woke-lite they have been slowly filtering into 5e.
After all, if people will buy a book with a combat-wheelchair accessible dungeon in it, what won't they buy?
Now, because of sheer inertia, it will probably still sell OK. But there will be a bunch of people who will buy it, regret it instantly, and then never buy another D&D book again.
As you said, due to the geek "completest" attitude, the next few books should sell ok.
This attitude will also slow any exodus from D&D.
D&D is a beloved game, and the Geek crowd tend to be very brand/IP loyal.
Many will be very reluctant to go to a different game even after the woke kicks into high gear.
So the decline will start after the next book from Candlekeep. It will be BECAUSE of the SJWs, and yet they will probably be able to successfully argue to the WoTC/hasbro execs that this wasn't the case ("candlekeep sold well!"), and that in fact the reason is because a new more Woke D&D edition is required. So unless Hasbro execs are way more intelligent than I think they are, the new edition will happen 1-2 years after the book after Candlekeep comes out. ...
I think this is a very good way for them to get a 6th edition made.
3e D&D to 4e was an eight year run.
4e to 5e was a six year run.
In 2024 at the 50th anniversary of D&D, 5e would have been out for ten years - two years longer than any other WOTC edition.
As the woke content goes up and the books sales go down post Candlekeep, the SJW's at WOTC will make the claim that 5e is on a downward life cycle and is beginning to get played out. Not because of "inclusive content". As that trend continues to play out they can easily make the pitch to launch 6e in 2024.
They will point to twitter and various internet fora (not this one) that going full woke is a must to keep all the "new players".
And if twitter is anything to go by, 6e will go in the opposite direction of 4e (3e + Heroclicks + MMO) and be more of a blend of 5e + Dungeonworld + Fate.
It will be interesting to see how this plays out as the SJW's at WOTC will have three years to make their case.