Open resolvers are like open WiFi, and open TOR nodes. They can all be misused, but I would still like to see them around in the future. A strict hierarchicy can be abused too, and it's not at all clear which is the lesser evil. I think you can solve a temporary problem created by an abuse of trust with another temporary use of trust... Ideally one could, if they were the target of a flood, control the flow towards them (egress filters) up the stream (the farther the better, which itself assumes a trust relationship) before it has a chance to concentrate.