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The whole point about Leon is that he has become depressed because there's nobody who can actually give him a challenge, to prove this the writers NEEDED to make a solid point by making Leon look unstoppable, making other champions look like novice trainers against him, this obviously made many people to hate the character and the series for "ruining" their favorite characters just to make Leon look strong, but that was a necessary step for the final.
why?
because they wanted to force people to think that Ash has zero chance of defeating him, this has 2 purposes
>subverting your expectations
by making everyone think Leon is way too strong, Ash winning will actually subvert people expectations and would create a "twist" or a "shock" ,however, the big problem with this route is that Ash's team is heavily underdeveloped, and it doesn't really make sense he can actually beat Leon's team that has been established as an isolated tier above champions while with the same team he barely wins against Cynthia.
>justifying Ash's loss
Now that they people have accepted the fact that Leon is beyond OP, nobody would really complain if Ash loses against him, in fact it would be only logical if he loses instead of his other losses where it seemed like they created characters with the purpose to cuck Ash from winning.
Honestly if this wasn't JN I would think they actually want to force Ash to use his reserves and beat Leon with his best team that he has trained of ALL regions, not only that alone would justify Ash's win but it would create actual hype for a battle that nobody seem to give a single fuck, but since this is JN, it will be either Ash losing against the strongest trainer in the world or winning due to massive plot armor.