I'd imagined that nodes would be able to pick their own ranges to keep rather than have fixed chosen intervals. "Everything or two weeks" is rather restrictive - presumably node operators are constrained by physical disk space, which means the quantity of blocks they would want to keep can vary with sizes of blocks, cost of storage, etc.
Adding new fields to the addr message and relaying those fields to newer nodes means every node could advertise the height at which it pruned. I know it means a longer time before the data is available everywhere vs service bits, but it seems like most nodes won't be pruning right away anyway. There's plenty of time for upgrades. If an old node connected to a new node and getdata-d blocks that had been pruned, immediate disconnection should make the old node go find a different one. It means the combination of old node+not run for a long time might take a while before it can find a node that has what it wants, but that doesn't seem like a big deal.
What is the use case for NODE_VALIDATE? Nodes that throw away blocks almost immediately? Why would a node do that?