Amazon having their own stablecoin could be really scary, same with facebook and microsoft and apple as well because these are HUGE companies and if they get any more power financially aside from just profiting from their work, they could be unstoppable and could become too powerful to bankrupt which means they will start doing whatever they want to do and if they fail simply they will be bailed out and knowing that they will do whatever they want with that freedom.
If these private companies can overcome the regulatory issues then there is nothing to stop them in coming up with their private centralized currency and the success rate of those depends upon how well people will be migrating into it and how profitable they can be minting their own digital money and maintaining the network, it is not an easy and lucrative business as we think it will be in my opinion especially if it is a stable coin and one miscalculation would take the company to bankruptcy especially in times of disaster.
I still think no private organization should be allowed to do stablecoins in the major nations, maybe small island nations and all but no major country should allow their big companies to build any stablecoin and should simply do government stablecoins.
Since this is a new market there will be many experimentation carried out in the next ten years, private companies coming up with their coins or token, countries coming up with their coins and many more drama is to be seen. If a company is able to hold an equal asset in accordance with the stable coin they are planning to come out, then why not.
. Imagine this, if Facebook's Libra which is one of the biggest companies of our era considered as a failure then what more for smaller companies. Government 1 - 0 Companies, 3 to go lol 
(just based on your list). 
