Survival mode will never be free, regular survival mode is temporarily free right now.
In the end only Creative Singleplayer will be available to non-paid users, and to us people who paid most of everything we will be able to access.
Your idea is to have Peaceful on /Indev/, so just buy the game.
Where did Notch say that in the end only Creative Single Player would be available to people who haven't paid? Because if that happens no-one will play the game anymore, thus reducing the amount of buys Notch gets. Or a ton of people will buy the game on the day that free people can only play on Creative SP, if that happens. 50-50 chance. I'm gonna say he shouldn't make Creative MP only for people who have paid.