I don't know, I haven't been watching anime in a while but I do remember Ash as an incompetent, yet extremely lucky, fool. I can't see someone like that winning.
Also if Ash wins then the anime loses his purpose. His entire goal is to become the pokemon league champion, to be the very best. In order to keep on with the anime our main
character shouldn't be achieve that goal until the very end.
Then again, maybe to goal is to become the actual champion, like the elite 4 champion. Anime makes it seem like being this ''tournament league'' champion and the elite 4 champion
two very different things.
Apparently, you haven't seen how competent and quick thinking Ash can be in this series. If he wins, the anime doesn't lose his purpose. In fact, he has another challenge awaiting him in Alola, should he be the one to go to that region next.
Furthermore, while Alain is enjoying the match, that's all he's been focused on. Battling Ash. Ash is focused on winning the Kalos League with his team, never giving up until the very end. Losing here would not only disregard all that's been said for him in the past, but would completely shun fans away in terms of character development and such.
And besides, who needs a loss more? Ash, who went through a string of non-victories and other losses, even with Ash-Greninja at his disposal? Or Alain, someone who hasn't lost since battling Siebold of the Elite Four. And keep in mind, Greninja was able to keep up with Charizard the last time, but Ash was unable to hold out for any longer. The battle could've gone either way, but still.