Let's say a consumer wants to buy something. They can either pay with "normal" money in one step or they can use their phone to convert their money into Libra and then make the purchase with Libra. Why would they go through the extra step(s)? Am I to believe that the benefit is "lower transaction fees"? Because if so, Libra doesn't have a chance. Only vendors, not customers, see those fees directly. Even if the customers did see them, they aren't big enough to sway the inertia of the way things are.
In the real world, the vast majority cares very little about things like decentralization. They don't care about how the banking system works and who controls the flow of money. They want to swipe their card and get what they want. Anything more than that is just an obstacle. At least BTC has the benefits of pseudo-anonymity and speculative appreciation. Libra has neither.
Beyond all of this, people like to have control over their money. To feel they have control, people need to have understanding. Paper is easy to understand. Cryptocurrency makes people nervous. Facebook REALLY makes people nervous. Put the two together and it will feel like a big risk to users.
I'm an open minded guy, but I don't see it. Someone please enlighten me as to why this won't be Facebook's biggest failure. Why would anyone want to use Libra?