I tell you what, if you're a young person you can be publicly shamed for having the wrong color bubble, or outcast by being excluded from a group chat. To you and I that probably doesn't make sense, but for a big portion of the young, phone using population being unable to participate in the iMessage ecosystem is a huge deal socially. You are essentially excluded from the friend group, you're excluded from conversations, and it carries over into real life because you've been excluded and that carries an impact -- you're an outcast. It doesn't have to be rational to be true.
Being excluded from a group chat has a huge social impact for younger people.
I've experienced this first hand, even as an adult (well 20 something). I'd be left out of group chats because MMS breaks everything. I'd even missed events because it was all planned in the group chat, and assumed someone would mention it in person to me.
Those young people are probably doing themselves a favor by disassociating from peers who are shallow and prejudiced enough to exclude someone for, of all things, not using a specific (proprietary!) chat app.
Oh come on, are you claiming you don't have arbitrary actions/clothing/language that you treat as a status signal amongst your in group? Have you forgotten what it was like to be a teenager?
Being excluded from a group chat has a huge social impact for younger people.