I think you would have to calculate watts required for average joe sending out 1 reply per 10 seconds from an 8 year old phone say a google pixel 1 or an iphone 8 and a spammer having to send out several thousands per seconds on optimized infrastructure (in which they'd have to invest as well)

I'm not sure what the numbers are here and maybe the spammer wins out in efficiency but running a 7950 whatever costs you several hundreds of watts while still having to invest in the hardware an energy.

It all boils down to if they can turn a profit from it or not in the end.