Even if Blockstack goes away, you can still access your data (in encrypted form) as Gaia is nothing but a storage system that is a wrapper over existing storage providers (aws, google cloud, and more). Now, the question is how you will decrypt it. Blockstack stores auth information on the Bitcoin blockchain so I guess you will be able to access your Blockstack credentials as long as Bitcoin blockchain is accessible and you have access to your secrets. Also, Blockstack and Gaia are open-source so it's unlikely they disappear with no trace.