That author constantly has drops in quality as series goes on, generally for a longer running series the art starts meh and gets better as the artist finds the style they want to go with and refines ideas, and then gets worse as complacency kicks in, I think the one that shows this off best is the ah my goddess manga, berserk kind of did this but it wasn't complacency, it was the mangaka taking long idolmaster breaks and then a switch do digital art, he never stopped trying but it is what it is.

Sometimes

I recently just dropped a series because the art changed styles, didn't drop in quality, just color tones weren't the same and the character design changed.

There are definitely series especially serial series where the art style slowly drops in quality as artist can't keep the initial premise quality. (I can think of many Marvel/DC comics that fall into this as well as shounen series.) I usually am fine with minor changes over time, but if its not that, then the story really needs to be good or quick to finish.

I find the art style to almost be pivotal to most graphic storytelling though. Like I don't mind both ONE's One Punch Man and Murata's, but I feel like they are in some ways telling a different story due to their art styles even though ONE's the lead author in both versions.

For example, Murata will draw multiple panels to show the strength and speed of Saitama's punch ability and he'll look heroic. ONE will be like this is punch and it looks derpy. So the gags hit differently. lol.

Internet Lurker At Heart

No. HxH and Mobile Suit Gundam Thunderbolt are proof of that. Both devolved into sketches for a bit but you can´t blame the severe health issues on the authors. It is what it is and both manga artistically recovered.

I also read EU/US comics and am a librarian.
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