You know, if you don't want Ash to win writers, that's perfectly fine.
But why sacrifice the narrative, story, character development and all of the bloody arcs in the process. Because of this battle, Goodra was a fall guy who barely did anything in the league, Ash-Greninja officially has had any development completely ruined because all of that training is undermined as we only got to see two full battles in the league. The furfrou guy from the first episode of the league is officially useless because Ash lost, and to add insult to injury, this sends out the message, you can win if you have power. Screw trust, hard work and dedication, let's win the gym badges in barely any time and beat all the peasants to get to Ash.
And the battle itself was anti-climactic. But there was always a doubt in my mind from the moment they showed off the Rasen-shuriken finisher in the previews that he'd lose. Why show off a finisher if it wins, takes away tension.
I genuinely hope the writers see the backlash and understand that Ash can still grow while winning a league. Because it's Ash, he'll always keep moving to a new region simply because of new pokemon, no titles are going to stop him.