I don't agree with this. I was on APforums, reddit and mangahelpers back in the day and I rarely ever saw any criticism towards One Piece but there were a lot of people criticizing Naruto, Bleach, Tokyo Ghoul, Magi, Seven deadly sins, Fiary Tail etc. Those were the top mangas back in the days but rarely anyone criticized Oda. So why was it that they criticized other mangas but not One Piece when the younger generation "boomer take" wasn't even around yet?
Hate in a manga has always been there even before the younger generation. People are now hating on One Piece more and more because it has gotten that bad not because of the younger generation of the Naruto fanbase switching over.
One Piece has been extremely consistent with the way it tells its storylines and if you were on AP, you'd know the same complaints existed even around Enies Lobby ("too much fodder action", "too much focus on luffy", "no stakes!"). The difference is, there has been a massive shift in the way complaints are accepted in manga forums and the fact that One Piece is now an "old guard" manga and no longer viewed as underrated of the Big 3.
No longer is it accepted to like something but also criticize it. Now you are either for or against One Piece. You are here to bitch weekly or are considered a shill. There is no longer a middle ground to stand on.
And i think mainly this stems from expectations in manga in general these days. Everything must have tension. Everything must have consequences. Shonen aimed at 12-15 year-olds must have peak writing. And a lot of this is formed by the new ground manga in general has broken.
Back in the day mangas like Dragon Ball had deus ex machina's (i.e. dragon balls) that could allow Toriyama to kill off characters and still revive them, or undo certain things that could leave lasting consequences.
Then One Piece came a long and barely involved death as a consequence, but when it happens its final. Then Naruto and Bleach came along and barely did it either, until Naruto started killing off main or big characters (3rd Hokage, Jiraiya, Neji, etc) or Bleach sort of doing the same thing. And these are just M3 examples. There are tons of shonen manga that started doing this. The difference is OP does it far less and always has.
So now the expectation for One Piece has changed. Other manga do it, but it continues to not do so out of very few instances far in between. So people become bitter and have been for years.
Tbh, I don't read other manga. Ive only read the M3, Dragon ball and Berserk. I have no desire to nor do I care. So my views on OP are very different from most people. I love it and always will. Do I think its perfect? Of course not. Do I think it satisfying weekly? For the most part yes. Do I get what I want out of it? Yes.