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.
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
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.
The fact that no one wants to bear the cost to pre-load it on devices, when combined with the fact that it's not nearly as easy to install OSes on mobile devices as it is on most laptop/desktop/server machines, means that it might as well be closed-source. The point of software is to be executed. If I don't have a good way to execute the software for its intended purpose, I have a collection of ones and zeros, and nothing more.
The window to have a real open mobile OS is starting to close. If there is to be a meaningful change, it must happen soon.