Can you provide your description of SAFE?
Is it not simply a protocol that eliminates servers and farms and thus provides a platform written in Rust to provide true decentralization? Then other apps, languages platforms can be written on-top of it but have the benefit of NO SERVERS.
There is actually no blockchain.
Tau needs a blockchain?
assume we build tau on safe and after some time we want to change safe's protocol itself (cause we found a bug or simply cause we have a better one) -- that wouldn't be possible, as safe isn't self-defining. that'd be one example. and yes tau needs a blockchain to contain the definition of the protocol at any point of time