She could’ve said something like “somi should practice her singing more” instead of downright trashing Somi

That statement would have been taken the exact same way as the actual comment has been, though. People are quick to assume the worst and embrace outrage, and a snide comment is a snide comment. There's a reason most idols aren't active on social media except on their official accounts and reply mostly to other members (if they're in a group) or post selcas with fanservice-y captions.

Idols (and individuals who want to be one) genuinely can't express a negative opinion like that in public. It will never end well for them, and it's something that will dragged up over and over again to support supposed attitude scandals over the next several years. That's especially true if you're a junior criticizing your senior, and in this case, two very popular seniors.

I mean, I agree with you. I guess I would just say that sometimes opinions shouldn't really be expressed at all if you're an idol or a potential idol. I mean, just in every day life, even, I've seen people say horrible things about "friends" who have sacrificed a lot for them, and it doesn't typically end well. People inevitably find out and are hurt by it, and it damages the person's reputation. Idols have a lot more to lose than someone who works completely out of the public eye.