Gentlemen, if I could butt in here..
Ixcoin, clone of bitcoin, has reached end of coin minting as of over a year ago. ix is a good test subject as it is struggling to maintain interest among the exchanges and miners due to not alot of transaction fees and zero coin reward.
Without a block reward, you are correct that the only incentive to be on a longest chain is so your transaction is confirmed unambiguously.
There are still transaction fees to consider (although obviously not in Iota), otherwise you might argue that bitcoin itself would suffer the same fate of diverging consensus, when the block reward expires.