The distribution of nodes is not the "only" thing that secures #Bitcoin . The decentralization of control over the special administrative privileges which allow users to write each block is what secures it against systemic exploitation & abuse from a central authority. That decentralization is achieved by coupling sensitive administrative privileges to power projection (hashing). By creating a global-scale physical power competition over those admin privs, the complex emergent effect is decentralization of control over the ledger.