If mining crypto is so profitable, what prevents companies like Exxon or Chevron to buy mining rigs, self-power themselves w their energy produced at near 0 cost while hedging with the futures? What would the bitcoin decentralization look like then? 

 They simply don't want things to be decentralized. Companies only get as big as they do in collusion with the centralized government.  

 I hope yakihonne remains decentralized as time goes on  

 Hope so too