assuming Ash wins, the only way I could condone his victory is if something happens akin to what happened when he won the OI league, Gary destroyed him and it made him realize he still needed improvement as a trainer, perhaps Leon would be a good fit for that role.
Or, have Leon show up and confront Kakui after the league is over, taking him on in Ash's place during the exhibition match and crushing him, and making him realize what truly goes into a league.
Of course there's the third option I mentioned in another thread, Ash could go to Galar, and if he mentions having won the alola league, then have people openly mock him as the Alola league isn't as credible as standard leagues are.