Your internet plan is already pay for what you use: you pay for a given rate of data transfer and your ISP maintains the infrastructure to provide you that rate. The cable you need for 50 Mb/s costs exactly the same whether you use it 2 hours per day or 20. Anyone downloading 100 TB in a month is already paying a hefty premium for the (at bare minimum) 370 Mb/s they need to achieve that download.
For electricity, in part you are paying for the infrastructure, but you are also paying for the fuel that's being consumed by the powerplant. If comcast charged per GB of data that it hosted, that would be fine (and hosting providers do exactly that without complaint), but Comcast's costs don't vary with the amount you download.
In the extreme, costs do scale to some degree with duty cycle - a line with lower capacity can service more people so long as they don't use it at the same time, and technically sending a packet through causes some infinitesimally tiny wear and tear. In practice though, a data cap doesn't stop multiple people from doing their 1.2 TB download all at the same time, and the reduced lifetime of components is a rounding error compared to the variability just from their manufacture.
For electricity, in part you are paying for the infrastructure, but you are also paying for the fuel that's being consumed by the powerplant. If comcast charged per GB of data that it hosted, that would be fine (and hosting providers do exactly that without complaint), but Comcast's costs don't vary with the amount you download.
In the extreme, costs do scale to some degree with duty cycle - a line with lower capacity can service more people so long as they don't use it at the same time, and technically sending a packet through causes some infinitesimally tiny wear and tear. In practice though, a data cap doesn't stop multiple people from doing their 1.2 TB download all at the same time, and the reduced lifetime of components is a rounding error compared to the variability just from their manufacture.