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.
I am proposing a design where the spenders (of transactions) are the miners. Thus if there isn't a lot of transactions, then there won't be much hashrate on the mining.
I have long stated that I am not interested in developing a coin that is distributed to HODLers. I want to distribute a coin directly to typical users of social media and other internet activities.
So my response to you is that a coin without transaction adoption is going to die. Why should it be any other way.