Precisely, the other 2 commenters are simply Alain/Kalos stans obsessing over the rematch.
Alain was undoubtedly a very big deal back in Kalos and I still consider Ash's performance against him his greatest feat (Until Journeys). Even above his Alola League victory or battle against Kukui which some people seem to rate higher.
That said, Alain from Kalos is severely outclassed by the majority of the Master Class challengers. Unless they further develop his character and establish him to be even stronger by perhaps defeating a noteworthy champion like Steven Round 1, it would be underwhelming to see Ash battle him when they are clearly better established and stronger trainers in the master class.
Plus, Alain as a round 2 matchup would still suck IMO as Ash would then potentially be fighting 2 back to back Charizard aces. Granted, G-Max Charizard and MCX are different, but still, it would be underwhelming given the number of options available.
Personally, I would prefer a variation of what
@Almighty Zard suggested where Ash and Alain battle outside the Master Class (Or Ash Greninja defeating a pokemon MCX doesn't defeat).
If the PWC is a direct knockout tournament with Ash potentially having only 3 matches, they could have an Ash vs Cynthia (Round 1 with the match against Paul being the training/preparation for this) and then have Ash battle against Steven/Diantha (The one who defeats Alain Round 1) in the semi-finals, and maybe here, Alain finds out about the vines in Kalos and offers to hold down the fort and allow Ash to use Ash Greninja for the next battle. Plus, in preparation for this battle, they could have a rematch between Ash Greninja and MCX.