The whole point of the first season/region/whatever was that there was a continuing journey. That every day, Ash would be stronger or wiser. That he was always progressing towards a goal. It was a reason to tune in every day. We saw Ash pronounced dead along with his friends and enemies in the bottom of the ocean. We watched his relationship with Charmander change and grow. By the time the big tournament rolls around, Ash isn't the idiot trying to catch a pidgey in his jacket.
Ultimately, Ash loses be big tournament. Which is kinda cool, actually. Teaches kids that it's not about winning. That the journey is what matters most. And he gets up on that horse again and keeps trying. He goes on new adventures and finds new Pokemon.
The problem is that, after some point, he stops growing. He starts regressing. He forgets how Pokemon typing works. He needs common things explained to him. He gives away all of his strongest assets for reasons that don't entirely make sense.
Eventually, he rounds up some of his old pals and takes on the battle frontier. And wins! He's offered the title of Frontier Brain, even! The best of the best! He's finally made it!
But then he turns it down. Ditches all of his war buddies with the exception of Pikachu and Aipom, because...um...okay...and then he forgets everything all over again. In the Darkrai movie, he's SURPRISED that THUNDERBOLT won't hurt Torterra. HE WAS OFFERED THE TITLE OF FRONTIER BRAIN AND HE CAN'T REMEMBER HOW HIS MAIN POKEMON'S TYPING WORKS.
Ash isn't a character anymore. He's just this weird obligation that the staff is compelled to uphold.
Fighting Red in G/S isn't awesome because you're fighting the guy you used to play as. It's awesome because he symbolizes all of the struggles you faced. Surpassing Red represents surpassing your own limits and being stronger than you ever were.
Pokemon is about progress. Collection, battle, it's all progress. Ash is the antithesis of Pokemon as a franchise.