The compression feature is awesome, should have been done a long time ago. However, it's optional isn't it? Because you mention Eventually, once the network is only running with new nodes, all blocks exchanged over the network will be compressed, which might not be the case if some/many witnesses choose or neglect to use compression. The network would still be serving uncompressed and compressed blocks, correct? Therefore, I think it should be recommended that everyone move to compression so the network/witnesses/endusers would benefit.