They didn't completely drop it all.
Misty's inferiority complex towards her sisters was there through out her time on the show. As for her development, they didn't drop it during Kanto. Her friendships with Ash and Brock helped her mellow out a bit and by the end of Johto, she is ready to go...
Your are conflating goals with backstory (which is pointless since Misty had a goal and her motivations were defined quite early).
Misty's backstory was clear from episode 7. She is the runt of her family and she basically leaves the gym to try to escape the shadows of her more well-known sisters.
Misty and Iris are not similar at all. Iris is actually closer to Kanto Ash in terms of personality, she is nothing like Misty outside of bickering with Ash.
Also saying Misty didn't have any backstory is BS.
SM is the most recent thing they did. That is more relevant then what they did from OS-XY. Also, it is not just SM, but their formula with movies changed.
Anyway, they already hinted at the next series being different. The fact that streamlined the promotion is a change from what they normally...
How I am I way off the mark?
They could have remained more faithful to SM games and yet they didn’t. The AU movies were a break from what they had been doing for 20 years.
Hell, how they are talking about the new series is break from what they done with each series since Johto.
I don't see how it is hard to believe there will be substantial change.
SM went away from the formula, as have the movies since 2017. Giving their track record the last couple of years, them doing a reboot or taking the next series into a different direction isn't a surprise to me.