> "Distributed filesystems and CDN in general are great, but let's use them for things that do not actually need a single bit of security, please."
The notion that distributed filesystems are inherently, or can't be, secure is way off. I would argue that with these technologies, such as IPFS, they can be more secure.
The use cases are not only "open databases" (by which I assume you mean open to public), private databases and data sets can be achieved just as well. Just because it's "distributed" doesn't mean it can't be private or access controlled.
Agreed on the comment re. "...illegal to share unless the user explicitly consents" and I believe this will turn out better in the trustless, distributed web, eventually. Our whole current approach is based on the client-server paradigm forcing us to put every user and their data into one massive centralized database. But we can change the model here. Instead, how about you owning your data(base) and controlling who gets to access it? "Allow Facebook to read your social graph?" "Oh, no? How about another social network app?". As a user, I would want to have that choice.
That bridges to your next point on authentication, which can be done on the protocol level with authenticated data structures. You can define who can read/write to a database by using public key signing/verification. It could be just you, or it could be a set of keys. One good example of this is Secure Scuttlebut (http://scuttlebot.io/). I highly recommend to take a look an understanding the data structures underneath.