Uhhh that's not even remotely true. HxH, One Piece, Prince of Tennis, Naruto, Bleach, Death Note all happened within years of each other. Throw in Gintama also if you want to since what it did for a comedy series is unprecedented.
If you go further back, Slam Dunk, Kenshin, YYH etc also happened with less years between them. The huge gap for jump to find the "next big thing" is after Death Note to Kimetsu although they did have stuff like HQ, Kuroko and AC selling a million a volume in between.
Out of the titles you mentioned only YYH, Slam Dunk, One Piece
Not even Rurouni Kenshin or HxH or... heck, even Naruto were on the level KnY is on right now (40million with 19 volumes which would put it slightly higher than 2mill pee volume, and even though the 40 million figure was probably across everything KnY related -aka the novels and other books- it will get to 2 million sold per volume.
www.comipress.com
KnY is pretty much unprecedented in how it exploded like this, sure we have the Dr.Slump example of the boost from when it got the anime, and also the One Piece 2011 boost, but it also isn't comparable because even though it got a huge boost.. it just wasn't KnY 15x levels of boost. (I'm guessing it sold around 100,000-120,000 pre-anime? And now it's at almost 2mill, so a 15x boost in sales basically out of nowhere.)
I wish we had sale data for Yu Yu Hakusho because I think it's pretty similar
Also Gintama isn't the first comedy to sell that good, Dr.Slump and Kochikame (in its prime, because it must have sold like hot cake back then to achieve the sales it has today. ) probably sold more than Gintama ever did per volume.
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I think a lot of people is just looking at total KnY sales (currently 40 million as of Feb 7th 2020) and those are impressive BUT when you realize that it achieved
2 million sales per volume right now (not years after it finished, thus selling millions in backlog thru the years, like most other manga data we have) that's way way waaaay more impressive because as I said it puts it higher than series like HxH and Naruto in per volume sales, in fact only DB, OP and Slam Dunk beat KnY in per volume sales. Maybe YYH slightly beat KnY sales too.
Now, what do Slam Dunk, Yu Yu hakusho and Dragon Ball all have in common? They all are from WSJ's golden age.
One Piece is an outlier, the best selling manga in history.
So KnY is selling at those levels (Lower than OP, DB and SD, higher-ish than YYH).