Ok so from what I learned so far the block chain contains the transaction history. And without the history we can't validate the blocks. So even if you want to clear the transactions that are already spent you would have problems because without them the blocks are not valid. Also when I began with BTC the chain was only around 14-15GB so it wasn't that big but seeing that now it's like 100GB+... even if we did a hard fork to clear transactions from newer blocks we'd have problems because of the number of nodes. Also I think that the 4MB block size is absurd but we surely can't live with 1MB anymore. I think the community should have the right to control the size but then number of those who are not having the full chain is high.