Why can't we just 'freeze' and archive the blockchain
Archive it where? Who is responsible for maintaining that archive? Who is going to pay for the archive? How will we know if the archive has been altered at all? If there are multiple versions of the archive, how will we determine which one is correct?
Special Archive Nodes. you know if the archive has been altered, because the hash is passed into the new chain. Any change to the old chain would render that verification hash invalid. If there are multiple versions of the archive, only the correct one will produce the right hash.
This way, 'common nodes' would only need a hash value and modest 150GB of local memory. Some special 'archive nodes' would keep the entire blockchain history and require many TB of memory. This way common nodes don't have to carry around for all of time everyone of the damn Satoshi Dice transactions.
Because the hash value from the prior retired chain is correct, a new chain can be sure that all derived balances are valid.