In the case of Dash, there is a built in difficulty and reward adjustment system. If miners leave and the hashrate falls, the adjuster will automatically increase the reward to attract more miners. When the miners come back the adjuster will level off the reward again.
Also, the unique nature of Dash's DAO means that we could even offer incentivized mining through the blockchain proposal system. Looking long term, the transaction fees should overcome any shortcoming as well.
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