This win was cool and all, but it was still only 3V3 and the anime had already established there are much, MUCH bigger fish in the ocean. There are still many undefeated rivals and plot threads to clean up, like Ash's dad. Heck, Ash has still never officially fought an Elite Four or Champion before.
Considering the next anime has all regions, and the soccer-ish theme of the next games, it makes sense to have Ash do one final victory lap in a serious "world cup" tournament, winning against those that bested him in previous leagues and bringing back old companions one last time to truly round off the anime. Then change it and bring something new.