He doesn't have to write Berserk.
A bit more seriousness. There are no newcomers to One Piece. I mean even Japan did polls and shit and most of the OP fans are people who started years ago and were growing up with this.
He won't lure too many new kiddos to 1000 chapters + manga.
Quite the contrary. The end of One Piece is closer than ever, so there’s probably more people getting into the series now than before.
If a story is good, people will read it no matter how long it is. Kishimoto seemed to burn out after a while (Naruto is still good) but we’ve gotten many good arcs like Alabasta, Thriller Bark, Marineford, Whole Cake Island, etc.
Just learn to go with the flow, and you won’t be bothered by things as easily.