not sure if it is needed to have the complete block chain to do a payment really
its not required, but you only know of transactions that exist in the portion of the chain you have. also, you can never know if the part of the chain you have is valid in comparison to the full chain.
IMO i think we need a drastic rework on how we make sure who has how much and such without requiring so much data to be stored forever.
i do not know of a way, but i would guess a way to not need the entire chain through "checkpoint" blocks. also we could drop addresses that have had no activity for x number of blocks.
or we could start over from scratch with a brand new genisis block and everything once the chain gets unmanageable. or even a competing chain would be good. that way the phase out period is smaller. (do not use ixcoin, we need a TRUSTED new chain, where the genesis block is made live, just import and start hashing)
this would also get rid of the "early adopter" "problem", since every few years a new chain would start competing with existing chains.