As a European, it's baffling to me that your friend group wouldn't simply switch to a messaging app with good group chat support. WhatsApp, Signal, Messenger (FB) - these are all great alternatives that are extremely popular, and all have more features than iMessage.
For example, in Scandinavia the current marker leader on all platforms is Facebook Messenger, despite this also being one of the only markets where iOS is the dominant platform compared to Android. Further south on the continent, WhatsApp is the undisputed leader.
People have all of these apps, and it's my impression that using iMessage exclusively is extremely rare. Cross-platform support is a feature that impacts which app people use, and they are perfectly free to use a different messaging app.
In the United States, if your friend group is using FB Messenger, all their messages are* readily pullable by police looking at someone two or three times removed from any one of them without a warrant “investigating” whether someone in their network asked a friend what to do about an unplanned pregnancy.
That's less likely to happen to you in Scandinavia, perhaps since Puritans found it safer to set sail for New England than keep trying to evangelize Vikings.
Also, yes, everyone's perfectly free to use a different messaging app, and in different cohorts everyone does, which suggests it's not Apple's "walled garden" causing these difference.
* This has been true at various times, including times FB has said it isn't true. I did see the WhatsApp founder walk away from a billion dollars over various issues, and stopped using it then too.
I don't really disagree with anything you're saying, except to say that, yes, the situation in the US is completely different than Europe, and nearly every time I comment about this situation, everyone who says they don't understand is European.
The fact of the matter is that in the US, especially for specific socioeconomic groups, iOS is by far the dominant platform, and for many people since iMessage is the default, their definition of "texting" is simply iMessage. You state, "it's baffling to me that your friend group wouldn't simply switch to a messaging app with good group chat support" - except that completely ignores network effects, and many times I am literally the only person on Android. If I'm, say, going on a vacation with friends and literally 10 of them are on iOS and I'm the only one on Android, it's a losing battle to try to convince the other 9 to switch, especially since nearly all of their other contacts are on iOS/iMessage. A restaurant in Austin, TX, famous for their funny signs, even had a joke about it: https://www.instagram.com/p/CwLKeGRLieb
The reason the situation in Europe is different is primarily due to 2 factors:
1. iOS was never as dominant in the EU as the US
2. Perhaps more importantly, what is often forgotten is that in the EU and elsewhere about ~15 years ago, per-SMS rates from telecoms were egregiously expensive, while in the US by that time they were mostly unlimited with a flat monthly rate. So there was never a huge drive to get off SMS in the US (this is why WhatsApp first took off much faster outside the US than within it). In a pretty brilliant "Embrace/Extend/Extinguish" move, iMessage initially simply because the default SMS app on iPhone. Users never really "picked" any messaging app, they just continued to use the "texting app" that had always been on their phones.
For example, in Scandinavia the current marker leader on all platforms is Facebook Messenger, despite this also being one of the only markets where iOS is the dominant platform compared to Android. Further south on the continent, WhatsApp is the undisputed leader.
People have all of these apps, and it's my impression that using iMessage exclusively is extremely rare. Cross-platform support is a feature that impacts which app people use, and they are perfectly free to use a different messaging app.