I'm trying to understand how can one create a cryptocurrency(based on ERC20).

You can manage every aspect of the currency(mint, burn, any conceivable feature, etc.) but at the end, when users use it: they have to actually spend ETH(gas for transactions).

Who will pay those costs? Why and how? How is that sustainable? even for such small fees, it isn't scalable. (Am I missing something?) 

Thanks!