But why should it stop? And above all, why should it have already reached its peak?
I can not understand ... the anime is still there, a game is scheduled, the merchandise is already starting ....
90% of series hit their peak and then go down to either stabilizing at a certain number or constantly reducing to lower levels. Even with an ongoing anime or media franchise. It does not matter. Take any well known series (Toriko, Bleach, SnS, Naruto).
Toriko - Hit it's peak in 2012, I think. But ever since then, the sales always went down, 20k-40k every 2-3 volumes despite the anime still being aired and having a movie, a game(the same year as the movie 2015 or 2016, don't quite remember) and being a good merchandise seller for Toei.
Bleach - Hit its peak of 1.26 million per copy but obviously even with the anime on going, the sales stabilized for a bit and then dropped after some time to around 750-800k before the anime ended. After that, we all know it went down even more every few volumes. The initial drop happened despite having movies, merchs, games, pretty much everything a franchise could have.
SnS - It's an ongoing example right now. The anime's next season is gonna air but the sales hit their peak and are now down from said peak.
There are exceptions obviously. But exceptions are usually something spectacular like One Piece or Kingdom or Shingeki No Kyojin or even Gintama. Despite all these years, Gintama has managed to keep a solid stable number for its sales. And aside from that Gintama's franchise just grew outside of manga and anime.
Of course, it doesn't mean BC's sales will go down. It might just stabilize around the current level until readers or viewers feel the quality dropped.
I don't see it increasing anymore until it animates something that makes people go "woah, that was cool" and generates interest among those who haven't read it or seen it yet. That's why MHA's season 2 helped because the initial arcs up until chapter 100 or something were top notch. That's why I say MHA will grow even more with season 3. But after that, if the mangaka doesn't step up his game to the quality of the previous arcs, it will go down.
This isn't rocket science, man. Just patterns and general trends.
On the BC topic, I'm fine with actual criticism since I can provide my opinion but here it's the same 3-4 dudes constantly whining as well as talking about a series that they don't care for. I heavily dislike SnK, Bleach, and Boruto so I never talk about them unless asked. How hard is that to do?
My discussion is about the sales. Not my fault some feel like they need to defend it even though I'm using facts that were presented few pages ago. If this was TPN instead of BC, I'd still be here discussing it because I like to talk about sales in general. People who've seen me comment on MH forums know how I like to talk about sales.