If Blockchain fails, it's back to centralised databases for everyone. At best, they might construct immutable databases. But, that's a poor fix for the "trustless, permissionless" requirement problem. Hence, some might be tempted to try out the half-way fix of the Hyperledger, and sharding, etc. But then, that's still technically "Blockchain". So, it's difficult to imagine how Blockchain might fail entirely, because there are layer upon layers of it's functionality; most of them being the ones that won"t disappear even if one or a few of the other layers gets discredited.