if we need 80%+ of support for everything, how can we ever hope to make changes to the fundamentals, like getting rid of wiki syntax in favor of rich text editing, or implementing a better discussion system?
The issue there isn't community support, it's development. There is work being done on the parser that might make implementing WYSIWYG possible in the future, but I wouldn't expect it any time soon. Once the feature is implemented, the community can decide whether to use it or not, but it's really a personal choice, so I imagine the developers would just implement it on Wikimedia sites unilaterally and let individuals use it if they like.