I like the idea of having quadratic pricing of fees per namespace, as this potentially allows fees per namespace to model fees of blocks more closely.
Rollups that try to sidestep this scheme by using many different namespaces would take on performance and latency penalties in terms of P2P topic discovery, block validation, and DoS/spam resistance.
I’m not sure that these penalties are significant in practice. P2P topic discovery and block validation could probably be optimized to be as efficient as having a single namespace, and DoS resistance might not be an issue with this mitigation.