Now the obvious solution is for the group to pick a cross-platform chat app and use that, but it's weirdly difficult to get a group of people to do that. Sometimes they would rather exclude someone from the group than switch apps, which might suggest that the excluded person needs better friends.

This is just objectively not true. The nature of green vs blue messages feeds into the in-group out-group dynamics that humans (children and adults alike) are prone to.

This is by design. This works well for apple.