The way Ash's current goal is set up, they can't reset him as easily as they've done in the past, and this could very well be his last hurrah as main character.
Even if he falls short of beating Leon, he might have to beat other champion or champion level trainers just to get to him, that alone puts him beyond going back to just doing gyms, since what would the point be? Yes one can argue that the potential gen 9 gym leaders would be tougher or as tough as champions...but there would be problems with that.
First off, if one was to make that claim then that would logically enforce Ash continuing to capture stronger pokemon out the gate (either already evolved or being quick to do so), or use his reserves, because if the gen 9 gym leaders were said to have that level of hype of being stronger than other gym leaders, then Ash needs pokemon to reflect this and not just going back to raising standard newbs that can barely fight from scratch, because let's face it, if they don't live up the hype they were given then why hype them up that way at all?
Granted I mean this anime's got a bad history of hyping stuff with no payoff, several times over...but to do that just to reset Ash is just as bad writing as some think journeys has now.
Secondly, if that were truly the case, then how exactly does a newbie trainer of the gen 9 region expect to beat them for badges? Origins Rules? Either way it would be stupid to do that to Ash after fighting at such high level combat for so long like he has this series.
I personally think the best outcome for gen 9 is Ash becoming a mentor for a trainer that wants to do the gen 9 gyms, like Brock and Misty (more so the former) were for him, while he continues to do the PWC in the background (either defending his title, or going after leon again and this time succeeding if he fails this round), and then gen 9 ends with that trainer having a battle with Ash... granted they probably wouldn't win but it would enforce the trainer to become stronger like Ash was.
of course that would only be unique 1 time, after that we would need an anime following the new trainer as ash passed the torch onto them, a timeskip with new characters or AU to keep things fresh idea wise.