What happens when the last block is mined?
It is clear that you have a misunderstanding about how bitcoins works.
"Miners" (or mining pools) earn a block reward.
That block reward consists of the sum of the current block subsidy (new bitcoins created) PLUS all of the transaction fees of all the transactions in the block.
At the moment, the subsidy is 12.5 BTC and the sum of the transactions fees is less than that.
However, over time the subsidy will slowly decrease (it will be cut in half approximately every 4 years). And as bitcoin gains popularity the value of the sum of the transaction fees will likely increase.
Eventually at some point in the future, the value of the sum of the transaction fees will be higher than the subsidy. There will still be a subsidy, but since the majority of the revenue will come from transaction fees perhaps we will stop calling them "miners" and start calling them what they actually are, "transaction processors".
Then the subsidy will continue to shrink until somewhere around the year 2140 (
more than 100 years from now). At that point it will shrink to 0 BTC and the entire cost of mining will be sustained through the transaction fees.