So, while Samsung may ship Tizen and Windows Phone, they may not build phones for Amazon (which caused Amazon a lot of problems back in the day - where will you find a company making portable electronics that will make them a white-label product and not making android phones).
2. MicroG depends on Google's servers, so Google can pull the carpet out of them at any time. And where will you get those millions of apps if Google doesn't let you access the App store? Look at the difficulty Amazon has at getting apps into its store, and it's a larger company willing to pay devs to switch apps to Amazon store. And all the hacks that people use will be closed by Google once they get popular.
Now sure, you can use MicroG as a porting library (so if I have a closed source app relying on firebase I can trivially switch to relying on MicroG and Mozilla's back end - kind of like wine), but you'll still have to recreate all of Google's apps anyways (there's no way Google will publish their apps on an Android fork) and also those apps written by those who don't care to port (unlike wine - where I can use the OEM CD or EXE from the OEM website, there's no official way to get apks out of Play Store)