There will be plenty of casuals. Most of the players in Vanilla were casuals and most of the players in Classic will be casuals. It will be a paradigm shift though and interesting to see how casuals today take it.
You will not be spoonfed epics. You will not be able to level to max in 2 days. You will not be able to gear up within a week. You will not be auto guaranteed to clear every raid. I was a casual in Vanilla. We accepted this. We knew we would not be able to do certain things and we were fine with working on what we could work on and progress on what we could progress on.