It seems to me that "no limits" scales when you can expect people to average out their usage. But when people just accumulate usage I'd suspect that it becomes

It seems to me that "no limits" scales when you can expect people to average out their usage. But when people just accumulate usage I'd suspect that it becomes harder to sustain. There has to come a known point where a user is costing more than they're paying - and when every active user is bound to reach that point sooner or later, I can't see it could be sustained.