Thanks for this Rob,
I stumbled upon this post reflecting on this same point for metrics in our industry.
I see CAC payback as an indicator of demand, distribution channel dominance, competition, but it cannot be looked at without looking at the context. Its a blind metric if you don't look at retention.
Insurance is a very clear example where industry dynamics make +12month paybacks sustainable. Having said that, it's clearly an aspiration to bring it down to 12 anyway. Any company with a 12 month payback on any channel would benefit from fast growth.