i dont know if this is feasible:
take a balance snapshot for all the address existed before a special block,and take the result as a new genesis block,then continue grow the chain from after this special block.the cost of this method is we lose all the transation detail before that point. and we can choose a right snapshot interval to do the procedure.may be it can be implemented as an option that users can choose.
hope guys give some opinion on my thought,thanks.
This idea is spread here every ~2 Months! No need for that solution
The answer to "why not" is that we don't need to. There are other solutions to the size of the blockchain, most notablely the one that was proposed by Satoshi himself in his whitepaper. The blocks were designed to be "pruned" of long spent transactions, if the user desired that, but never to require such an action. That's why the internal block structure uses a Merkel Tree instead of a flat file format. Including special blocks to update a running list of non-zero balance addresses would not only require breaking the current system, it would also eventually be blocks of such massive size as to be counter-productive.
There is no full client that utilizes pruning that I know of, but there is nothing preventing you from coding that yourself if you have the skills and believe that pruning is a near term need.
Furthermore, anyone who desires to avoid the blockchain altogether can simply use a light client, such as BitcoinSpinner or Electrum.
Search the forum for "pruning" of blocks, light clients, and "Stratum" for more information.
