As @ardos commented - we can export worlds (and even subsections of worlds).
We don’t have a rigid plan for the current worlds from here to 1.0 and beyond. These worlds are the environments that we’re using to help develop, test and iterate features and the game against. We will try hard to make sure they’re stable and long lasting, ie. we’ll not do unnecessary wipes. (The team has already worked pretty hard to keep these worlds live. You don’t always see the work they do to migrate and maintain them between updates. You only see the downtime when the migrations don’t work smoothly 
)
If there is sufficient demand - at a minimum we could allow players to download the worlds if we needed to take them offline. This would allow players to keep their creations and play them locally. “Download my beacon” also feels like a relatively simple feature to add.
As ever - let us know what you want, campaign for it - and we’ll try and make it happen.
Finally - we’ve never wiped a world yet.