According to Sotoshi's white paper, it seems that a 51% attack would only disrupt current transactions but not affect the previous blocks.
Yes, and people seem to forget that these attack blocks still have to be accepted and propagated by regular Bitcoin nodes throughout the network. There are 2 possibilities:
1) The attack blocks violate the Bitcoin protocol. In this case they will not be relayed by regular nodes and they will immediately orphan out
2) The attack blocks do not violate the Bitcoin protocol. In this case they will be relayed by regular nodes and make their way through the network. However, because the protocol is obeyed the scope of the attack would be restricted. E.g. the exclusion of certain transactions from the block.
In any case, although transactions would likely be lost the disruption would be temporary. Miners will eventually leave the compromised pool and things will go back to normal.