blockchain – Suppose crypto is adopted globaly, and 100 years pass. Where would 100 years worth of global transactions/smart contracts be stored?
[…] each computer has the entire chain downloaded onto the hard drive? Or do we merely query the chain from designated nodes?
Full-nodes (a node is a wallet etc) typically store a full copy of the transaction journal (blockchain) locally in non-volatile storage – typically a hard-disk. Not in memory (RAM).
Pruning nodes discard transaction data that has been processed and that they no longer need – this greatly reduces their storage needs. It does slightly hamper other new full-nodes processing the full transaction journal – they have to find other, non-pruning, sources for pruned blocks. They need to do this to independently and trustlessly determine the current state (the set of monetary amounts valid for use as inputs in future transactions)
Lightweight nodes, mostly those that use Simplified Payment Verification (SPV), trust other nodes to store the full transaction journal. They don’t store a permanent local copy of the transaction journal to any significant extent.
In future it may be that, in the next 100 years, SPV nodes will predominate or that non-volatile storage will be smaller, cheaper and higher capacity, or that changes to the network protocols etc will address this in novel ways.