Mining is required to verify transactions.
If there are no miners, then there will not be any confirmed transactions, because no new blocks will be added to the block chain. If that happens, Bitcoin is dead.
However, miners currently receive TWO incentives for mining:
1) They earn 50 BTC for every block found (which halves every 210,000 blocks on the blockchain)
2) They earn transaction fees for all of the transactions confirmed by the block they found.
In theory, as miners exit, the difficulty will be reduced, so it'll be easier, not harder, to find blocks. When #1 is no longer attractive, the theory is that there will be so many transactions confirmed by a block that miners will keep mining just for the transaction fees.