Did people seriously think the writers cared about Misty/Brock? After the original Kanto season ended, the writers didn't even care about Misty/Brock much when they were actually still on the show.
Considering it's also been nearly 15 years (!) since Misty was permanently dropped from the anime way back in 2002, it's rather obvious they weren't going to go back to her after nearly two decades of her being absent from the series. However what they should have done is had Leaf the female character from FR/LG as Ash's companion, and not create this new girl with the Piplup.
Charizard took about six years since his last appearance. And besides which, they waited ten to thirteen years for Jessiebelle to make a reappearance. By your logic, they should never have returned, period, because it took so long for them to do so.
Also, I might as well point out that the fact that she got merchandise sold like hotcakes last year and even this year with those Gachapon preorders shows that she was marketable as well, meaning that even if the writers didn't care for her, they are obligated by sales to showcase her nonetheless.
And besides which, it's the 20th anniversary, they clearly marketed it for the audiences who would have been old enough to remember the first episode, and then pulled a fast one there, that's a jerk move regardless of whether the writers liked her or not. Heck, both Misty and Brock, alongside Serena, even made the top three characters people want to see return in a certain Japanese poll, which took votes from both 10-19 year olds and people from their 20s, meaning both people who were actually getting started on Pokémon and those who were there since the beginning, and everyone in between, meaning there were plenty of fans, both old-school fans AND younger fans alike, who wished to see Misty and Brock return (in fact, Brock ranked first place, and Misty ranked third). Heck, I also heard that even the Japanese fanbase was outraged at the writers omitting Misty and Brock and took to Twitter to express their displeasure (and in fact, this is I believe the second time they've done that in recent history, the first being the infamous Kalos League results).
Let me put it another way: Dragon Ball: The Path to Power, the 10th anniversary dragon ball film, did something similar, where they retold Goku's first journey. They included all of the old Dragon Ball Gang from the time of the original journey, even Yamcha, who is infamous for basically being the resident punching bag of the group (and it's pretty clear the writers have no care for him since they generally depict him as a weakling). In other words, even if they didn't care about Misty or Brock, they could have at least been respectful of their audience enough to include them all the same. I know if I were the writers, even if I didn't care about Misty or Brock as you claimed, I'd STILL include Misty and Brock in the movie regardless of my own personal feelings precisely BECAUSE I know the fans want them and it is appropriate to use them due to it being the 20th anniversary, and I'd even know they have market potential.
And for the record, the writers attempted to give Misty a lot of Pokeshipping hints during the Orange Islands, and even Takeshi Shudo in his novelization included a Pokeshipping hint in there, and they even gave her plenty of opportunities to continue onward with her goal. If they truly didn't care for her at all after Kanto, why keep her at all after Kanto, much less attempt to give her more limelight and even focus up on Pokeshipping? They could have just as easily done to her what they did to Brock in the Orange Islands alongside Brock himself during that time especially if they truly didn't care about her, especially when technically, if respect from her sisters was all she wanted, they'd boot her after either Princess vs. Princess or The Misty Mermaid (both of which dealt with her actually gaining respect from her sisters). No, the fact of the matter was, most of the writers did in fact care for her. Maybe not Takeshi Shudo or Masamitsu Hidaka, but definitely most of the writers.
What's your problem with those characters, anyway? Every time many of us are justifiably angry over the writers showing blatant disrespect over them, you make snide remarks about how the writers don't care and we should just sit back and forget about it. If I didn't know better, I'd assume you hate the characters themselves and actually are glad they aren't returning. I mean, even a lot of people who otherwise were glad Misty's doing something they consider of value by being forced into the gym are upset over Misty and Brock not returning, even saying there was no way they could have predicted this, yet you if anything are cheerfully taunting us on how the writers don't care about them at all, which suggests to me that you outright dislike them and want them erased. And for the record, I actually dislike Brock especially Johto onward where he was becoming more of a Quagmire clone, not to mention unlike Misty makes very little effort to actually focus on his goal in any way, yet even I don't have it in me to outright erase him, especially when he ultimately played a huge role in how Ash became the trainer he was today, and if anything am disgusted that the writers have such a disrespect for the show's roots.
Anyways, going by the plotline, well, Cross might have been Damian done right, which is probably one of the few things the film gets right alongside Ho-Oh actually receiving focus. Unfortunately, most of the film beyond that was crap (Gary and Ash barely even interact).