What was the reaction back in the day when Misty was abruptly written off the show and a new girl quickly replaced her in the new series? Seems it would be kinda jarring, especially as it was initially portrayed as a totally new start for Ash, both Misty AND Brock leaving, so it didn't seem like an 'attack' on one individual character. But then Brock showed back up 3 eps into Advanced. Kind of a bait and switch. So really it was just Misty who was killed off. How did fans react to this? Did they just take it in their stride, or was there a backlash. I don't see how it wouldn't have been taken as an 'attack' on a beloved character they grew up with, seamlessly replacing her with a new girl.

I've read that the reason they did this is because they thought a character refresh would be more profitable, but of course Ash and Pikachu were too fundamental to the series to change, so they changed the girl sidekick instead. In retrospect, were the execs correct in this, or did it make no change, maybe even adversely affect commerical success?

I always thought that kids often turn away from things if characters change, kids are autistic and want things to stay the same. I personally turned away from Power Rangers because I genuinely resented Jason Zack and Trini being replaced with 3 dweebs and Tommy getting ridiculously buffed (new White powers + taking the leadership role) - and that was well before I learned about the actual irl shenanigans that led to that change.
I also quit wrestling because after Rock and Austin basically left, I wanted Jericho and Angle to become the new top guys, but they were used as enhancement talent, and eventually they went with Batista and especially CENAWINSLOL, two guys I hated. Apart from two specific matches (shawn-vs-taker at wms 25 and 26), I didn't watch any other wwe for over 10 years after wm21 (when Cena and Batista were crowned as the new champs). I remember being thoroughly disgusted after that ppv lol.