I really find it interesting that manga has managed to keep in times with it's artistic development and yet not leave out an old fashioned amazement of beauty with everyone saying : WOW, I LOVE THAT ART or WOW THIS ANIME HAS WONDERFUL VISUAL AND SOUND ASPECTS TO IT!
As I recently was listening to some anime music, this Fairy Tail opening in particular got me really excited to see how aesthetics are always high in manga, in every aspect. In manga the art and writing is high level, though some of the writing falls short of the art. But The anime are always complete with directing, voice acting, animation. it is wonderful. Even anime that appeal to us because of it's ugliness have a high aesthetic appeal, some horror, sci-fi as well.
Why do you think over the years manga/anime never lost their traditional appeal yet managed to keep up with the times and be as modern as they can each year? It appears that the aesthetics of manga are never lost.
The character designs didn't determine what was popular, but rather it was styled after what was vogue in that particular time period. (SEED might be an exception.)
Interestingly Toriko has an old-fashioned appearance compared to everything else from the late-millennium (in terms of shonen) but is extremely popular.