the problem with this argument is that Cynthia was an established champion from the get go, where as in Iris case we don't have an exact time frame on when she became one, that kind of plays a factor here in how different they are in terms of experience from a long term perspective.
I still think by doing so is why he managed to beat her, if had used any other pokemon there would've been no hesitation on her part, but between Dragonite's feelings, and Dracovish surprising her, (and unnerving her Dragonite) that is why she lost.
I mean when other type specialists like Gardenia and Olivia are shown to be capable of restraining themselves during battle against the elements they use and love, but Iris clearly couldn't, and that is pretty telling in of itself, as it shows that Iris still needs to discipline herself more so that her love for dragons doesn't cost her battles when they are used against her.
I actually partially agree and disagree with what you say.
I don't think Age/Experience really matters that much. Leon is much younger than Cynthia but reached a level even above her in a much shorter time frame.
That said, we clearly see Caitlin from the Unova Elite Four, hold her own against a Cynthia who at the very least was as as strong as she was during the Sinnoh Episodes, if not more. I think it's fair to say, Iris at this point is as Strong as Previous Regional Champion Tier and Cynthia is now one step above that as the PWC is far more challenging and difficult competition versus the Traditional Regional League and Champion's League.
She lost because champions have been nerfed now. If a dracovish who was born yesterday could defeat iris then sceptile or greninja would have a field day with her.
I disagree. I think it is the opposite where the Regional Champions in the Master Class are much more stronger versus the Traditional Regional Champions who we see in the Previous Series. Considering the fact that they are setting up Ash to Challenge the Strongest Trainer in the "World", when he was always shown to be much Inferior to an Elite Four/Regional Champion in the past, it only makes sense for them to immediately bring up Ash to a relative level to make his final battle against Leon far more believable.
And as for the argument about a Pokemon with less experience doing well etc. This inconsistency has always been a part of the series. Like lets even take one of Ash's strongest Pokemon (Ash-Greninja). Its best feats before his battle against Alain was beating Wolfric and Sawyer who are by no means, advanced or very powerful trainers. Alain's Charizard on the other hand, has battled and defeated over 20+ Mega Evolutions and on top of that, it was the first Pokemon that actually defeated an Elite four's signature Mega in a serious battle. It even held its own against a Regional Champion's Mega Evolution.
I mean, Ash had Froakie while Alain had a Mega Charizard and challenged Siebold to a battle. Despite the jump, we still see Ash Greninja put up an even fight against Mega Charizard X.
If we go back to Sinnoh, let's look at Aaron's beautifully. Despite him just getting Beautily years after becoming an Elite four Member, he uses it against Cynthia and it seemed relative in Strength to the rest of his team.
I think it is fair to say at this point that Ash's team will simply scale up based on where he stacks as a Trainer. At this point, Ash is already a Trainer who defeated a Past Regional Champion tier trainer. So at this point, I think whatever Pokemon Ash catches, will scale up to be relative to this level far more exponentially versus some of his older mons from previous regions.