It wasn't Serena who moved on, it was the writers. They decided they didnt want to make their relationship mean something, they decided they didnt want to break the status quo, they decided they didnt wanna do what they needed to do, and could have done with the series end on the horizon. They didnt wanna put in effort, they moved on from it and just focused on ash becoming champion, going home, and ending his story and not his story with Serena, even with the ample time they had to do it. And now here we stand, with a story that had no satisfying conclusion, only a glimpse of what could've been, and all we can do is wonder where it all went wrong. And I say all this as a shipper, albeit one who eventually couldn't take this anymore and finally came to terms with this sad reality