I think that’s where we’re blocked. The only costs I can see is channel bandwidth and battery life. Channel bandwidth relates to your comment about “measuring really continuously, i.e. not every few hundred usec, but perhaps every atto second” and has already been addressed. And I’m talking with a “fundamentals of meter design” hat on here, not about any specific implementation.
About multiple channels Vs de-aggregation of the mains signal? I’m a long term advocate of per-breaker monitoring. Everyone I know with a smapee reckons it reports their dishwasher comes on when they’re hoovering or something similar. de-aggregation is a hard problem to solve, and good luck to them for trying. Hopefully it’ll improve over time, but for now per-breaker CTs is hard to beat.