The basic rule is you own your data. This includes the design of your application and the data that your users upload (pursuant to your own agreement with them, of course). What Bubble owns is the code that runs your application, which is not app-specific. This is similar to the fact that Microsoft owns Microsoft Word, but does not own the books written with it.
We support automatically exporting user-created data in the form of CSV files. You can also use the Bubble API and write some scripts to read your data in an automated, more scalable way.
Finally, we want to make sure you feel secure about building on the Bubble platform, so, although we plan to be around for a very long time, here's our guarantee: if for some reason we ever have to close down shop, we will release the Bubble source code under an open-source license so that you can set up your own Bubble server and keep your app running.