Word of mouth isn't exclusive to the period AFTER launch or launch window. If I watch a trailer a month before release, talk to someone else with words from my mouth, and that causes said person to watch the trailer and then want to buy the game, that is word of mouth advertising. I'd imagine there was plenty of that considering how well it sold compared to entries in the Warrior series.

Without opining about the quality of the game or what the word of mouth is: it's perfectly possible for a bad game to sell millions of copies. That's what hype is for, after all: those same reaction videos feed into selling copies before the game is reviewed or released, the initial interest was strong at least in part because this was billed as the prequel to one of Nintendo's biggest games in years, and being Nintendo's only serious holiday title didn't hurt how many units were moved. I think we can all name games off the top of our head which were quite bad, but still sold millions on copies on the back of the marketing machine. Even E.T., The Worst Game Of All Time™, sold 1.5 million copies in an era where a tenth of that was a genuine blockbuster.

Now, opining about the quality of the game, and speaking just for myself: it was meh. I haven't finished it. I'm not sure I'm going to finish it. I'm not a big Warriors fan, so that's not a shocker, but I was hoping that at least the game's other elements (e.g. narrative, characters, etc.) would make up for it. They didn't for me. No one really interacts much with one another, I haven't learned much about the world or events that I couldn't piece together from the base game, and the characters and events aren't even the ones we glimpsed in Breath of the Wild, so it's a bit of a bait-and-switch anyways.

I can't speak to word of mouth, since I haven't had my finger on the pulse for years, but I have a hard time imaging it was too spectacular, if the sell-through numbers have dropped so greatly.

Oh, right

Bad word of mouth after launch*

Problem solved 

Most of sales are for pre existing that had the switch for ages and fans of breath of the wild.

Dontcha know

You really don't like the game or the genre, don't you?

Just FYI, it got a 8/10 in user score on Metacritic, slightly above the score from reviews (7.8/10). Doesn't really look like bad word of mouth, now does it?