Yeah but right now, due to the right choice of foils and situations, Ash is still kept well balanced in spite of said flanderization. I dunno how he would fare if Brock and Misty were his main foils again, or vice versa (Misty always targeted Ash's ego, for that she would need Ash's ego to exist at all on a regular basis, and when Ash didn't need a straight man as much, Brock slowly dwindled in interactions).
A lot of characters' roles and importance start to wax and wane when the rest of the show doesn't bounce off of them correctly anymore (eg. Team Rocket).
It depends purely of writing, actually. In a world of
fair writing, I can see current Ash not actually that fond of how he was in his Kanto days (that's why sounds out of character
now for him lying about his first badges like in SM Kanto episodes). Kanto Ash was a slacker and wouldn't miss an easy win. Current Ash, despite how he changed along the years, is consistent about be humble, train and win fairly (even if training in SM consist about twerking, what
bizarrely works).
Dynamics with Brock would be mostly the same, since he followed Ash's gradual change into a good trainer (who still sucks in any other non battle related department, though), and since everyone in Alola is chill as heck + lots of girls, Brock would be in a paradise (eh, no pun intented). It isn't as he was in need of a foil years ago. If anything, Kiawe (in "manliness") and Mallow (in cooking) would serve to that role. Misty is other case, her main thing was press Ash's buttons and nag him down. While he still need to be called off some times, Ash doesn't need so much of that nowadays, since he is, to all effects, a better battler than her, with temper or no. Again, no one in Alola is that confrontational, and while Misty can be bratty, she would feel out of place. Technically, they could focus only on her water thing and make her feeling at home.
Personallly, I don't miss Brock or Misty or Kanto shenanigans, not even a little bit (but, Kanto pandering, LGPE promotion and all). I'm expecting they're comming for a short time (three episodes at most) and then, bye.