A few years ago, when I was a teen in high school, I was part of a decently large group that used iMessage to make plans and hang out. We never had any Android users who consistently showed up, so iMessage was an easy default.
I recognized that we could potentially be excluding people who didn't have iPhones, so I brought up the topic in the group. Switching to a regular MMS group was a non-starter -- MMS is slow and unreliable, administering large groups is a complete disaster, and many teens can't receive MMS messages at all because they don't have a data plan (whereas iMessage works over a Wi-Fi connection). So I researched alternatives and suggested we switch over to Slack (this was before Discord got big, so Slack seemed like the best option at the time).
I got flamed hard for this suggestion. People very much did not want to download a new app, make an account, and learn to use it just for one group chat. iMessage was working great for us, so we decided to stick with it and reconsider if an Android user actually wanted to join the group. (One Android users eventually showed up, but the group went with the path of least resistance and just relayed information to him separately rather than switching chat platforms. Of course, this meant he missed out on all the socialization aspects of this group).
Eventually, we hit iMessage's 30-person limit on the number of participants in a group chat, and so we were forced to switch to Slack. This was a complete disaster: Slack was cumbersome, intuitive, and a painful transition, and it completely killed the group chat. It wasn't worth opening up a new, clunky app just to send a meme or joke to some friends, and Slack's extremely unintuitive notification settings [0] meant that nobody saw anyone's messages anyway. It didn't help that this happened in the fall right after some people had moved off to college and a bunch of new freshmen joined the group; and within about two weeks the group ceased to exist.
My takeway from this was that the iMessage lockin was definitely real, not because of any social stigma but because iMessage was, at that time and place, far superior to any other well-known product for our use case. Everybody made a Slack account and switched over to the new group, but nobody used it because it was unintuitive and cumbersome. Nowadays it seems Discord is finally succeeding in this space; I'm in college now and among my peers Discord is by far the preferred means of communication in small and large groups alike. Even many of my professors are on Discord and use it as their primary communication channel outside the classroom (I've had several classes where office hours are held in a Discord voice channel). Its interface is cleaner, easier to use, and more performant/responsive than Slack's, its notification settings have sensible defaults but are easy to customize, and its voice channels are an absolutely incredible feature that puts products like Zoom to shame.
I know Discord certainly has it's fair share of issues when it comes to freedom, privacy, and interoperability -- but ultimately teenagers don't care about any of that -- as you said, we just want to talk to our friends. It's all about who offers the best features for the least friction -- and I'm not talking gimmicks that nobody uses like cryptocurrency integration or talking animated avatars, but rather functional aspects including the product's reliability, usability, practicality, performance, and of course the network effect. A few years ago the winner was iMessage on nearly all fronts, today it's Discord.
[0]: https://pbs.twimg.com/media/C6ROe0mU0AEmpzz?format=jpg&name=...