This morning I was talking about this with a buddy of mine.
Right now we have miners all over the world trying to solve this exponentially hard algorithm to earn their bitcoins. That is their incentive and that is what drives them to do so.
What happens when we remove that incentive to "mine" aka "verify transactions" on the blockchain? What I'm basically asking is what happens when we hit 21,000,000 bitcoins from the perspective of the miners?
Then all the bitcoins available would have been mined and then there would be nothing left to mine, so all we can just do is to keep confirming transactions as miners then, and earn just a little from transaction fees.
By that time miners will be so energy efficient that they will be able run hardware with very low energy costs and in that way they are going to be able to depend only in the transaction fees that they receive from the users that make a transaction, besides it may be possible that the devs change the hash algorithm to something else if most of the mining power moves to segwit2x.