1) We could maintain fees through compression, that is, by making each on-chain transaction representative of many off-chain transactions. This is beginning to happen with the lightning network, and other technologies are in the works to advance this concept.
2) Mining will become more integrated with other industrial and residential processes, which will reduce the cost of security, from a compensation standpoint. Think bitcoin miner water and space heaters in many homes, etc.
3) It's also likely that in the future institutions that rely heavily on bitcoin will voluntarily subsidize security to some extent, for the same reason they invest in vaults for other instruments. Personally I'm planning to start running a lottery miner at home just for the fun of it.