Remember when people kept justifying Android over Windows Phone/Maemo/WebOS/BlackBerry/FirefoxOS on the grounds that it was free and open source software, infinitely customizable, and that Google was a good-faith partner who wanted openness in the mobile market?
Good times, good times.
AOSP is still open. The problem is nobody wants to bear the (monumental) cost of polishing and convincing brands to allow installing it in THEIR devices. Google was motivated back then for creating an alternative and openness was a good bait.
This is exactly why I switched to Android. Running any Linux binary with just a small bit of patchwork to get Xorg running (this was before Wayland) felt like magic. So much power in your pocket, I could plug a keyboard and display into the device and use it as a computer! And internet-connected 24/7! Coming from a literal Nokia where I made some web-apps for the javascript-supported browser (that was already a major leap for a mobile phone), it blew my mind. The sole reason Android blew up is the endless possibilities developers saw (starting with the device manufacturers of course). Now the developers are here, the competitors haven't been competing because who cared about Firefox OS or Windows Mobile even back in 2014, and so Google can do whatever