While I agree that breaking it up into smaller age ranges makes sense, Disney has massive kids clubs with elaborate theming around Star Wars and Marvel. 11 and 12 year olds still want to use those facilities. Now they are in a much smaller space that is mostly just a lounge for playing video games.
Our other concern of course is simply the timing of the announcement. We did a European cruise this summer and we booked Princess over Celebrity based solely on the age grouping to ensure our kids would be in the same one. We didn't know about this change for
DCL until after we'd booked and paid in full.
I emailed DCL and at least suggested that they provide an OBC for those families that are now having our children get split up. I realize they need to make the change sometime and it can't be perfect, but at least soften the blow with some $ for family activities.