When Ash was a new trainer, people had no problem with seeing him defeat trainers that were clearly more experienced, yet now the boot is on the other foot, they don't like it..
Some people are going to start their journeys later and catch up with and/or surpass Ash, just like he still progresses by catching up with and/or surpassing people stronger than him. That's how being a trainer works, heck that's how you progress through a region, by defeating people far more experienced than yourself (gym leaders?).. Would you rather have another Trip who unexplainably became this unbeatable fellow right off the bat, or would you rather have someone like Sawyer who slowly but surely improves and gets to the point where he can rival Ash without being randomly o/p from the get-go?
As said, he's already got Sceptile to basically the same point as Greninja, where they are on equal footing until AshGreninja comes into play. Then there's an inevitable Salamence which will go from punchbag to monster, simply because that's what the species do as they evolve.. and the same again with his latest capture which will easily bulk up his defences through evolution. Plus whatever else he's training whilst storming through those gyms at the rate he is..
I see a good battle happening at the league and if that ends with Sawyer's victory, then I'll be more than happy with it because like I said, Ash isn't the only person who should be able to surpass someone. Remember when his regional team defeated Paul's mix of reserves? Yeah..