There is nothing that supports that he will use reserves, and in case of using them it will be in master class, not here, If the directors want Ash to win with a new pokemon he will do it, do not underestimate that
nothing to support he won't either just because he hasn't used them yet. With all the patterns this series has broken so far, I don't see it as being too far out there, sure he might not use them, but he still could we don't know what the writers will do in the end.
Problem is his reserves can only be fall guys, unless his current team doesn't participate in a battle at all (Which makes their captures pointless). Just face it, Ash isn't gonna use his reserves at all during the PWC. If he was going too, he should have done so from the start and give his reserves spotlight (Especially the non-aces), but then he wouldn't have gotten a new team.
Most of the trainers in the first two tiers were not worthy of his reserves, and it would've felt like overkill to employ them, now that he's in the ultra class, their time to be used is now if the writers so deem it, since he's up against opponents that actually are worthy of tasting their power.
After all, there's no need to use your big guns until you really need em, heck Ash could've used his Z-ring much earlier, but waited until now when things got tougher, you could apply that logic to his reserves as well.
Here lies the problem: If Ash uses his reserves alongside his current team, his reserves (Especially the popular ones), are gonna overshadow Sirfetch'd, Gengar and Dracovish and those are exactly the three mons that are craving screen and battle time.
I don't think most would care if they were overshadowed given how i've seen numerous comments about the "poor construction" of this team compared to previous ones, especially in terms of camaraderie.
I mean heck the sinnoh league which was one of the best leagues for Ash, other than Staraptor and Gible the rest of the team only got to fight Paul.
I agree that Sirfetch'd and Gengar do need more combat....Dracovish i'm not so sure...it's first win was against Iris Dragonite, and that was one of the biggest wins i've ever seen a pokemon get.