blockchain database - why so redundant
See https://en.wikipedia.org/wiki/Blockchain_(database)#Decentralization, which states that "Every node in a decentralized system has a copy of the blockchain. This avoids the need to have a centralized database managed by a trusted third party."  

This is as true of 10 computers as it is of one.  It's not about redundancy from this perspective, though that is certainly a motivating factor.  It's about having zero centralized authorities,  and the only way to do that is to allow everyone to have a copy of the blockchain.