Facebook currency will become a reality because there is nothing one can actually do to stop the technology from being released, they have every right to apply what will benefit them, but it will never gain the market dominance as we think, do you know how many countries that we have in this world, and each of them have their own policy, and so far they still have law in place that the citizens fear, then nothing to worry about Facebook coin.
Before Facebook coin can really gain that popularity in any country, then the government of each country must have accepted it which like you say will be impossible because everyone wants to protect their own currency, in fact starting from china and India alone, Facebook coin is already a failure being that they have the largest population in the whole world.
We may not be able to stop the technology from being released, but the government could do so at any moment. After all, we've seen how the US has been concerned of Libra's legitimacy within the mainstream world. President Trump has expressed his ideas of Facebook's cryptocurrency, where it can be used for various illegal activities. Regulators from various countries have bashed against Libra, as they're afraid that it might replace the world's monetary system.
Believe me, many governments are aware how powerful Blockchain technology is. Facebook's aim of becoming the world's next-generation central Bank with Libra coin, will prove to be a threat to the existence of Banks worldwide. That's contrary towards the proposition of governments as they want to gain control over their people's finances. Central Banks want to remain in power, which is why they'll try to do anything at their disposal in order to destroy Libra at all costs.
Luckily, that won't be the case for other cryptos like Bitcoin and Ethereum because they're highly decentralized and censorship-resistant (unlike Libra). With their distributed nature, and ever-growing support from people within the mainstream world, they become a force to be reckoned with. Libra has a higher risk of failure within the long term, because of its single point of failure (Facebook). If it becomes a success, then it could bring a greater level of adoption for Blockchain tech within the mainstream world.
Nonetheless, Facebook would need to go through various regulations in order to establish itself as a central bank. And since it aims to become a global financial institution, it's going to become even harder to comply with all of the world's regulations. Which is why, it's very unlikely that Libra will launch by 2020. Just my opinion