First of all, this is in no way an attempt to bash people who do headcanon this! It's a totally fine headcanon. I just don't personally subscribe to it.
But as for why I don't like it, I honestly think it makes Bobby seem like less of a kind person, because it almost is implying that Bobby is only kind and understanding to the girls because he's been abused by the industry/was a rejected Kpop trainee, not because he is simply a good person who wants to take care of his charges. I know Bobby is a supporting character at best, maybe even a minor character, but I feel like his character is stronger if he is just a good guy because that's the type of person he is.
Honestly, if there's anyone who had a bad manager, it was probably Celine and the other Sunlight Sisters, especially given how in the early 2000s, I doubt it was looked highly upon for a woman to become pregnant out of wedlock in general, especially if she was a Kpop idol. I more headcanon that Celine went out of her way to find a good manager for the girls so they didn't have negative experiences like she had.
And on a side note, I really, REALLY hope that in the sequel, they don't write Bobby out and give the girls a mean manager. That would be such a lame, kids-movie, sequel-y plot point. I've seen people suggest that the bad manager could be demons in disguise, but even if they were, that's such a cliche plot point. Across the Spiderverse has shown that Sony can make actually GOOD sequels, so fingers cross we get more Across the Spiderverse and less Surf's Up 2.