Theres literaly no problem in keeping his target audience around 15yo, the problem is that the overhaul quality of the story dropped significantly since the timeskip when compared to pre ts. The story already went into some pretty dark themes for a 15yo target audience, with slavery and what not...
The target audience has nothing to do with the problems this series face, you can tell he is tired of writting the same characters for all this time, thats why he has been shoving new characters and giving them focus instead of the crew...Who the fuck cares about scabbards, momo or fucking furries? Dont get me started on how retarded he was to split the crew for two whole arcs... Just give the og crew some quality fights instead of the disrespect they have been getting.