Yes, One Piece coming in a lower place has less to do with the decline of One Piece than it does the truly incredible sales of these new series. One Piece only ever did these types of numbers in the years around Strong World's release. As Captain M says, people are going back and buying all the previous volumes for series like JJK and Demon Slayer.
One Piece is being driven entirely by new volumes released in this calendar year and Oda's publication pace has necessarily slowed down.
So we can really just say good for these new series without worrying about One Piece. It's not pulling the same numbers as it did at its peak, but it's also still undoubtedly incredibly popular, particularly for such a long running series.
I'm actually most curious now to see if the Live Action series could create a new boom in sales, perhaps even in the west if the show is a big hit. The odds that live action One Piece will be as big as Game of Thrones are incredibly low. I mean, I'd love to be wrong. I'd love to see US audiences as enthralled with the voyage of Monkey D. Luffy as they are with 'Baby Yoda'. I can dream.
The live action series is probably still two years away. Maybe it can be the next Stranger Things size hit for Netflix.
I'm just curious to see what, if any, the potential success of the live action series could have on manga sales. In the US, comics and manga are a far more niche industry and I don't get the sense that the MCU has had a tremendous impact on Marvel Comics. I do know that A Song of Ice and Fire became much more popular through Game of Thrones but I wouldn't say anywhere close to a majority of show viewers also checked out the novels.
If the show does succeed, Viz & Shueisha should be ready to go with some deluxe 3-in-1 type books. That's actually what I'm most hoping for from One Piece. Also the digitally colored editions in print.
But at the core of it, this is much more about the phenomenal success of these new series than it is the decline of One Piece. The latter might grab more headlines, but it's less descriptive of reality. One Piece is still selling incredibly well. It received a huge boost in popularity with Strong World and the Marineford arc which buoyed sales to 34(?)+ million in its peak year, which at the time was unprecedented. It reigned at the top of the sales chart until the past couple years; a decade+ long run as number 1 in annual sales is still unprecedented and it's still the overall highest-selling manga of all time. One Piece outsold Spider-Man. Demon Slayer has surpassed One Piece in terms of average sales per volume (6.5 vs. 4.9), but that's also the one title One Piece never claimed from Dragon Ball.
As for anything else, I'm sure Shueisha isn't panicking or even upset by One Piece seeing a slight, gradual decline in sales. One Piece sales have gradually declined since the big wave created by Strong World / the Paramount War. It seems to have settled into a 7 - 10 million pattern. If you go back to the mid to late 2000's, sales numbers like what One Piece is putting up now would have still placed it near the top of the annual sales chart.
What this means is that the success of JJK and KnY is not coming at the expense of the 'reigning king' so to speak. It suggests that the manga industry as a whole has expanded and that these series are bringing in new readers. That they were able to so completely dominate the sales chart without One Piece suffering a precipitous decline in sales suggests only the sheer magnitude of their success. This is a win for Shueisha. They have their super dependable 100 Volume long series still selling 7 - 10 million books on an annual basis and they now have clearly discovered a successful pattern for new series. They don't have to worry about finding the next One Piece or even Naruto level success. JJK proves KnY isn't an anomaly. As Captain M said, it's a winning formula which doesn't rely on series like One Piece, Naruto, Bleach, and Dragon Ball which are designed to last for hundreds and hundreds of chapters.
KnY ended after 23 volumes and from what I understand, JJK is unlikely to run for a long time. MHA is clearly rushing toward a conclusion sometime in probably the next 50 - 100 chapters (at most). From what I understand Dr. Stone is also close to ending, leaving Black Clover as the longest consistently running series in Jump (not counting Hunter x Hunter) aside from One Piece.
I'm of the opinion One Piece still has about 11 - 12 years left, which I know is not particularly common given Oda's comments. Personally, I have really loved everything from Zou through Wano and this is the most I have enjoyed weekly One Piece since CP9 through the Paramount War. Oda's clearly enjoying writing his story. A modest decline in sales for a 100 volume long series isn't going to raise any alarms. The fact that the series is still reliably selling so well is a strength, not a weakness. And they still have cards to play that will probably increase One Piece manga sales again in the future. I already mentioned the live action series. When the series does eventually approach its climax, the end of One Piece is going to be big news. And they can also reboot the anime, whether that's a full remake like FMA Brotherhood / HxH 2011, or an edited version like DBZ Kai remains to be seen.
It's a weird case where the current anime has stuck to the story of One Piece unlike FMA Brotherhood. One Piece has consistently run in the magazine, so the anime wasn't forced to end like Hunter x Hunter. So it's harder t justify a full remake of the series. At the same time, the first two story sagas in the anime are very clos to the pacing of the manga. Without much fat to trim, it's hard to imagine a DBZ Kai style reboot having as much appeal. Not to mention, One Piece transitioned from 4:3 to 16:9 during its lifetime and there are dramatic pacing issues and some rough animation in later story sagas. There could also be some competition between an anime reboot and the live action series considering they would be covering the same part of the story. I'm not saying they absolutely wil reboot the anime, just saying it's a card they could play that would boost manga sales. My bet is that the current anime series will end 1 - 2 years after the manga finally concludes, which means a solid decade if Robby is right and One Piece ends in 7 years, or even longer if my estimate of 11 - 12 years for the manga is correct.
I would bet more money on a sequel or some type of anime-original story set in the world of One Piece like Boruto following the conclusion of the manga. They'll let time pass and remake the One Piece anime sometime farther into the future when several years have passed from the conclusion of the current series. They'll make it high budget, seasonal, and start from the beginning.
The last point I will make is that Naruto sold about 7.4 million copies in its peak year, 2010, and declined to 3+ million in 2015, the year it ended. One Piece, in a 'slump' year with only 3 volumes, is still putting up comparable to Naruto at its peak. And Naruto is a series considered to be a mega hit by Shueisha. One Piece is not in any danger and any predictions of doom or even concern are not really justified. Sheuisha is still spending a ton of money on promoting One Piece. Think about all the fanfare surrounding chapter 1000, volume 100, and episode 1000. There's a new film next year, an upcoming live action series, and as discussed, there are ways they can perpetuate the success of One Piece even after Oda finally finishes creating his masterpiece manga. No one is going to rush Oda along or urge him to change the course of his series to capture sales / an audience that aren't his to capture. A series has to play to its own strengths. One Piece is 100+ volumes long and it would be ruined by trying to capture the same audience as Demon Slayer and JJK. One Piece just needs to stay the course until it reaches the end of its voyage, which I hope is not too fast approaching.