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There's literally no reason to believe Serena isn't endgame.
He's spending time with Misty? Great. Misty is literally his best human friend, Misty isn't like Serena, Ash isn't physically intimate with Misty while Serena literally kissed him.
We were meant to assume Serena and Ash get married in the future so that still stands if the anime ends without suddenly switching gears to Misty.