That's a hell of a generalization--one that you can't possibly prove. Any author from any country is equally capable of falling into seasonal rot if the story goes on long enough: just look at American television and how many good shows went bad or simply mediocre.
It is a very, very well establisbed characteristic of the Japanese comics industry that comics keep going well beyond when the writer and/or artist has run out of ideas so long as the books are still selling, and they keep going until the books stop being profitable. It's somewhat of a different phenomenon than what is seen in American comics, where runs do end when the story is complete, but the comics are set in shared universes, so the same character eventually gets a new run with potentially different artists/writers to continue the story of that character in a different scenario, possibly with a different supporting cast. I recognize there are comics outside of DC and Marvel, but they were never as popular and they often shared the model.
The problem with American comics is that, in the 90s and early 00s, many of the old guard dropped out of the industry for one reason or another (or were let go as the comics industry was shrinking with the rise of gaming and TV shows with continuous narratives since the industry no longer had the cash to pay for them), and the new blood has never been able to capture an audience of their own, largely due to their inexperience and/or lack of talent. That is to say, both industries have their own sets of problems, and they are different problems from those the other is facing.