The problem with internal/external attacks is that we (the society) don't really want to prevent it. The reason is simple: child porn. To date, Bitcoin block chain (and related ideas) is the only data-storage that is 100% resistant to attacks (i.e. changing history), but luckily it cannot handle amounts of data large enough to be viable for child porn (or most other forms of media). Tor, on the other hand, gets a bad rep precisely because it doesn't prevent it (despite its numerous other, beneficial, uses).
The core of the issue is that humans view different information differently (child porn vs. Mona Lisa), whereas for computers, bits are bits and numbers are numbers. As long as child porn remains illegal and socially unacceptable, we'll want to enable attacks on data, i.e. for someone (usually internal operators) to be able to delete some kind of information, corrupt it or at least track it. Of course, this necessarily means that all information stored in the same data-store will be vulnerable.
You're conflating the archival properties of the medium with the decision about what to save. Oil paint on canvas is durable. It doesn't mean that a museum needs to retain every piece of crap that anyone paints.
The problem is that removal of content because it's crap/immoral versus operator destruction is not a meaningful distinction, from a software perspective.
So it would probably need to be write-only to prevent people from burning it down, which would necessarily mean that, once content is included, it cannot be modified or removed.
The core of the issue is that humans view different information differently (child porn vs. Mona Lisa), whereas for computers, bits are bits and numbers are numbers. As long as child porn remains illegal and socially unacceptable, we'll want to enable attacks on data, i.e. for someone (usually internal operators) to be able to delete some kind of information, corrupt it or at least track it. Of course, this necessarily means that all information stored in the same data-store will be vulnerable.