There's a reason why JRPG series don't usually stick to the same character casts. Each entry tells a story and things wrap up in the end. Sometimes, there are stories covering two or three entries, but those are rarer. Examples to consider: Tales, Final Fantasy, Fire Emblem, EarthBound, Dragon Quest, Xenoblade Chronicles, among others.
Same goes with the Pokémon series with several adaptations (mainly manga) following the same models... Except the anime adaptation, for some odd reason. And it shows why the quality is deteriorating. The story and characters have become stale, the whole thing has dragged for too long.
But in the end, sales and quality never run parallel.