"The only true number that matters is aggregate peak demand. If aggregate peak demand exceeds network capacity then packets are going to drop. So if the ISPs were being truthful and selling real services instead of fictional ones, they would sell plans with bandwidth caps that kick in only during times of congestion"
You 100% got it. Cost to ISP's is about gigabit/sec peak not gigabytes per month. Only addition is some types of traffic are cheaper due to POP caching e.g Netflix caching servers.
A competitive ISP structure leasing last mile to POP as suggested would allow for much better indication in ISP choices between caching, transit purchases and peering. Also clever models based on customers who would pay premium for capacity at peak, latency etc.