Then it would just be Ash and Cynthia being protected by the plot, same difference, like Gladion and Alan were also protected by the plot to be able to face ash if we're being technical. Furthermore, by the nature of stories being artificial, this will happen regardless of how well written a story is, so it doesn't matter really.
The issue, as you've said, is that these trainers are too strong to be treated as jobbers without affecting suspension of disbelief on some level, But this issue wouldn't be fixed by having Leon sitting out imo, it would be by simply making them put up a fight, because while I don't doubt Leon is the strongest undefeated c̶h̶a̶d̶ champion, I find it hard to believe he can just steamroll champion lvl (even E4 lvl) trainers without trying.