You make a good point, that it is vital not to exclude people who only have a mobile. An ERC-20 token isn’t the only, or necessarily best way to achieve that though, and since inclusiveness is an essential goal of Safe Network, it will be a very high priority once the network itself is functional.
Anyone with a mobile running Safe Browser has a way to interact with the network, so on the most basic level people could earn by carrying out tasks for others. At some point there will be a Dex on Safe. In-between I expect MaidSafe and the community to look for other ways. ERC-20 is one way.