The league doesn't end the generation. We've had stuff after leagues before, after Kanto's League Ash went to Orange Island to do stuff there until the next generation of games came out. After Hoenn's league Ash took on the Battle Frontier which were located in Kanto, again this was done until the next generation of games was ready to come out. And now Ash is again doing stuff after the league until the new generation of games comes out, only difference is this would be the first time Ash stays in the region instead of going to another place.
Originally Posted by Crystal_Knight
Also, the only reason Team Plasma wasn't in the earlier episodes of Best Wishes was because of an incident that happened that postponed their original debut. The writers just decided to have them appear later on and have them be the closing of the 5th generation for the Anime.
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