Yeah. I've figured that a model where you can earn "free money" by walking, will not be sustainable in the long run. Maybe letting users earn minuscule amounts of money (pennies?) over long periods of time, will work? But the underlying cryptocurrency would need to have real use cases for the mainstream world, in order to retain its value for the foreseeable future. As you've said before, MangoCoinz failed because it lacked a sustainable economic model for its survival. Not to mention, it was behind other cryptocurrencies in its marketing/promotion efforts.
If by any chance we're able to see something like this again, it might be in the form of an ERC-20 token. It's much easier for a startup or average Joe to reward people via an existing Blockchain network than making everything from scratch. But I guess that there are other more important things to focus on, besides letting people earn crypto by walking. At least, the idea is there. Making it a reality is another story. With so many ways to earn money with crypto today, there's no need to create a coin/token that would reward users by doing nothing other than walking. Just my opinion