I think it just bothers him that One Piece out sells any other manga even Naruto by a factor of 10.
It is since a couple of years the most popular manga ever, even surpassed DBZ
It has sold 350 Million+ copies world wide, which is the current record. And it is most popular on the 5 biggest manga and anime markets of the world namely: Japan, Indonesia, Philippines, France and Brazil. (Also the countries surrounding them but these countries are the biggest and represent the market the best in those areas)
I also took the liberty of finding out how popular One Piece is in the US, because i know how much narutards like to brag that Naruto is the biggest over there. And that is true. Well it WAS true, because Naruto lost massive readers after shippuuden and One Piece only continued to grow. So even on the American market (Source is Q99 sales ratings) One Piece has surpassed Naruto BY A MILE.
*1, 15,220,095 One Piece
*2, *4,178,598 Naruto
*3, *3,616,942 Fairy Tail
*4, *3,169,048 Fullmetal Alchemist
*5, *3,029,300 Nodame Cantabile
*6, *2,626,932 Bleach
*7, *2,533,556 Kimi ni Todoke
*8, *2,376,060 Gintama
*9, *2,083,508 Katekyo Hitman Reborn!
10, *1,574,448 Bakuman
One piece doesn't have the impressive stats as in other countries, but still outsells the number 2 by a factor of 3.
Since 1999 One Piece was always on #1 as the best selling manga of the month with the exception of 5 months. So let's do the math here: 1999 to 2014 is 15 years x 12 months = 180 months. So it was the best selling manga in 175/180 months XD
You call One Piece childish but you probably just don't like the early One Piece anime graphics, which again were created 15 years ago, probably when you still were a little boy. The One piece story also tackles subjects that other mangaka dont want to touch with a ten feet pole, racism, discrimination, ethnical cleansing etc.
Also look at this graph, One Piece has a way bigger adult fanbase because they stuck with the story so long and apparently like it enough to still be reading manga at a older age:
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So basically in all aspects, manga sales, popularity, story, humor, art and everything else..
One Piece smashes the competition, it's not even funny. And probably that's why you don't like it. You hate that you can't get yourself to like something that the rest of the world enjoys.
This pretty much sums up why all the hype around it is actually based on something. 15 years of manga ans till going strong. Too bad for you that you're not a part of it.