Yes. Mean girls at school are mean. If everyone has the same color bubble they’ll just find something else to be mean about.

This is really reductionist. This isn't a bullying or in-group / out-group thing.

Someone who is not on iMessage will be excluded from iMessage group chats, just as someone who is not on Snapchat will be excluded from Snapchat group chats, just as someone who is not on Instagram will be excluded from Instagram group chats, someone who is not on WeChat will be excluded from WeChat group chats.

This is correct. Everybody has green bubbles in Europe even on iOS, because everybody is using WhatsApp. But mean people are still mean.