If the transaction fees from miners are not sufficient to maintain the security of the ledger, than the network will self-destruct.
But, with the cap being so-far away (who knows how many years) by then computing power should be sufficient to ensure the sanctity of the network with very very minimal costs (low power costs), it wouldn't surprise me if bitcoin takes off and becomes somewhat of a world reserve currency, that most "smart homes" or devices, in that time frame will be "bitcoin enabled", and will donate small % of cpu cycles to maintain the network for the good of society.
Think about it, at that point who cares if mining is not profitable? when your fridge is...mining at the cost of 1$ USD a month (in todays money).
All the computer power will come from small powerful low power usage devices that will cost pennies.
Decentralization, all the mining and such that is going now is just baby steps for bitcoin (or its replacement), the money flowing into bitcoin (disguised as "profit") to miners is really just the world slowly building a monetary base to support the coin and development of the network.
Free-Market economics based on profit will evolve out of mine to profit into "mine to mine for the sake of keeping your savings secure", just like how we let banks charge us absurd interest on loans for the sake of making the transactions somewhat secure and keeping the money system "going"....however in-efficient and draconian the current economic model is.... atleast it works ...sorta.
Bitcoin is star-trek level money, we need to reach global prosperity while minting the new monetary system into existence.
People worried about the "end of bitcoin when it hits 21m" are very near-sighted.
The above is based on my view and basic understanding of economics....