Backwards compatibility is still quite important in major version releases. Just look at Angular 1 to 2, Rails 2.3 to 3, and Python 2 to 3 to see examples of this.
I'd say (and probably this is what you meant) that those are examples of what happens when backwards compatibility is broken - the community gets fragmented, finding answers on StackOverflow gets more difficult, lots of helpful blogs/articles/videos are now useless, etc.