The number of transactions are limited by block size though so each completed block has a maximum number of transactions it can include.. also securing the blockchain isn't an end unto itself it's only really useful when it includes transactions.
It's not that simple. As I recall the last time a block size change was proposed it fractured the entire community and produced a splinter "currency".
An example would be that me not driving to the beach saves some fuel but bitcoin is just a train constantly driving to the beach and back regardless of how much I use it. You could grab the number of trips and the power usage and calculate a power usage per trip but its not an entirely useful metric since the power usage does not scale up as more people use the service.