I've been hearing the new "Big 3" discussion for a while now and it's funny how people keep trying to push this notion without any real consensus being made. I think there's even been talk about a "Dark Trio" of shonen manga, consisting of Kimetsu, Hell's Paradise, and Chainsaw Man.
Back in the day when One Piece, Naruto, and Bleach were the Big 3 it felt more natural and was kind of a perfect alignment. All three began publication within a few years of each other, all three were published in WSJ, all three were incredibly popular, and all three ran for 10+ years, becoming mainstays of the anime community. With the current era, though, things are a bit different. If I had to select a new Big 3, it would probably be Kimetsu, MHA, and JJK. All three were published in WSJ within a few years of each other and have been mostly successful, but beyond that things get kind of complicated.
Kimetsu is incredibly popular and financially successful, but its manga only had a four-year run, so right now everyone's just looking forward to how the anime plays out.
MHA was also very popular and a had a ten year run, more than twice Kimetsu's length, but I feel like people have gotten a bit disillusioned with it during the second half of the story.
JJK also has a very successful anime adaptation, but it seems to me that the series is very divisive as people either love it or hate it. In any case, the manga is set to end in five chapters, meaning it will have run for a respectable six years.
So yes, all very popular in their own rights, but also relatively short-lived, at least compared to the previous Big 3, and in just over a month all three of their manga runs will be over. So pretty much any interest people have in the series now is down to their anime adaptations, which I guess is always the case for a lot people anyway. Of course, there's also the fact that most anime nowadays, KnY, MHA, and JJK included, follow seasonal releases, as opposed to One Piece, Naruto, and Bleach, which all ran year-round. And while I'm certainly all for seasonal releases since I think it overall creates a better product not bogged down by filler, it does create a different environment for anime viewers. Rather than tuning in every week, anime viewers now wait for the new seasons as big events.
Anyway, I'm kind of rambling at this point, but to summarize I think it comes down to the fact that more series these days are content to have shorter runs, which is all well and good, and that I think there's just more competition. I didn't even get into how One Piece is still running, Spy x Family and Chainsaw Man have both been pretty popular, and Black Clover...well, I don't know how popular it is Japan, but personally I struggle to see how anyone could even consider it at this point.
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