...except that the last mile has the same 10mbps or 50mbps or whatever capacity even if you're on the crippled-speed budget plan. The only cost to increase the quota is for the ISP to upgrade its major inter-site links, not the edge connections.
If even 10% of the end users fully utilized their circuits they would be far over their capacity to get those packets to their core, even if the core could handle moving them to other networks. As such, increasing the amount of transit the ISP purchases may do nothing for allowing it to be effectively utilized at the end user location.
The last mile limits bitrate, not quota.