As far as I'm aware, few people talking about Network Neutrality were ever talking about metered usage? The concern has always been about traffic discrimination

As far as I'm aware, few people talking about Network Neutrality were ever talking about metered usage? The concern has always been about traffic discrimination based on destination. Indeed, even QoS optimisations need not violate Network Neutrality if done in a destination neutral way, e.g. using the IP QoS flags. You could offer someone 1Mbit of dedicated low latency bandwidth out of their normal higher bandwidth connection, and let them use it however they want.