Winning the league yet losing to the "Champion" of the region is an excellent way to get the point across that Ash still has a lot to learn despite his victory. And even if Torracar beats Incineroar there is the chance Kukui has a back up in case his ace fallsThere's literally no reason to have Ash lose. He already won the League...
Besides it'll be incredibly stupid for Torracat to be the one that gets screwed over yet again, not getting to one up his rival while Lycanroc and Rowlet did.
Only way I'd accept an L for Ash at this rate is if some Galar trainer shows up in the last episode, and mostly wins because of surprise as Ash doesn't know what Gigantimax is at this time.
He already lost to him and Tapu Koko before, many times.... time for him to beat them.