I don't really understand how anyone can truly believe that Ash defeating the entire PWC, including Champions like Cynthia and Leon, using only his JN's team would make any sense, narratively speaking.
In past series, they made it clear that the gap between regular trainers and Champions/E4 was huge. As an example, remember when Ash's Infernape had nothing to do against Flint's Infernape, and that was near the end of DP, so Infernape was at his strongest. And it was Ash's ace in DP.
The only pokemon from Ash that ever achieved impressive feats were ones that were returning pokemon from past series. Like Charizard defeating Articuno in AG, or Sceptile defeating Darkrai in DP. Plus Pikachu, of course, since he's always there. And the only pokemon to really compete against a Champion was Ash-Greninja.
So, sure, you can say Ash wants to try to get through the PWC using the team he's built this saga, and that's fine, but then he sure as hell shouldn't be getting any further than the first Masters 8 trainer he faces with that team. Otherwise, it's just bad writing.
If they want to make it believable for Ash to become the number 1 trainer in the world, he has to use reserves. If not, they're just retconning previously stablished facts.