The real problem is that when miners are competitive they'll always add cost (in electricity, equipment and whatever) up to the level of the revenues. So if you

The real problem is that when miners are competitive they'll always add cost (in electricity, equipment and whatever) up to the level of the revenues. So if you have a block reward - it will approximate the cost of the system. There is a famous post on this subject: Nothing is Cheaper than Proof of Work http://www.truthcoin.info/blog/pow-cheapest/ http://www.truthcoin.info/blog/pow-cheapest/ (even though I believe that Proof of Stake can be cheaper: https://medium.com/@zby/proof-of-stake-can-be-cheaper-than-proof-of-work-da14223a965 https://medium.com/@zby/proof-of-stake-can-be-cheaper-than-p... - by reducing the block reward).