Clothing (or other things, clothing was just an example) does get you excluded from a group. The only reason a bully would want to "include" the bullied person in their group is so they can bully them more.
I agree that they could open up iMessage to competitors with relative ease and that this would be a good move. Not because it would seriously stop bullying, but because it would make it a little bit easier to find a common messenger to use (we don't really have that problem in my home country, as most people use WhatsApp, which is multi platform).
What I'd hate is if Apple removed all indicators that what I'm sending or what I already sent is an SMS/RCS message instead of an iMessage. It shows me what features work for that particular conversation, and if I'm roaming in a region where sending SMS is not free, I want to know when I'm about to send one.