Alt coins have different coin caps, so they will not "run out" at the same time. ^smile^ The whole Crypto currency protocol was designed to sustain the network with miners fees, once the Block reward runs out. The Lightning Network will have Lightning hubs that would also serve as a source of income, when the Block reward stop.
Satoshi thought about everything. ^smile^
Well, the incentive to mine will be now on the fees. By the time that happens, the fees will be worth as much as block rewards are today.