Interesting take, that’s a really good way of looking at it to be fair and makes the appearances of all the characters logical.
Above any reason for why Ash will beat Leon, this is the main one. Above any power scaling, above any powerup usage, above anything pertaining to Leon's character. Once Goh left to finish PM, Leon's fate was sealed as far as I see it. If this theme is going to stick the landing, Ash and Goh will complete their JN-objectives at roughly the same time.
At the very least, it means both Ash and Goh either win or lose. There will be no one victor between the two
It's also the reason why anyone else being in the stands is ultimately noise. Ash remarks that Leon's got the same species as Goh's Pokemon - this battle is connected to him. It's going for that type of parallel. You can replace Dawn and Chloe with literally anyone else - Serena, Misty, Todd Snap, anyone - and the outcome and significance of Goh will still be the same. Dawn's only here because she's the person closest to Chloe, a current protagonist, so she has someone to talk to while Hop hypes up Leon, I believe.