If thousands of vendors had made something Android 2.X level for the entry level market and the technical people who actually want what Google engineers wanted the ecosystem by volume would have been mostly FirefoxOS instead of mostly out of date Android and the ecosystem by revenue still Apple's.
Aside from all else, and what about the very responsive Windows Phone 7 or similarly flawed WebOS? Let alone all the feature phones, Blackberry etc. This was an enormously competitive era, and Google had to do some serious fighting to secure the future of Android at that time.
Phone manufacturers do not exist to serve the bidding of the software builders, they actually have to make money, and doing that requires looking at least superficially competitive. The primary way users experience that is through the graphical interface on the device, hence why my then employer made incredible amounts of money selling game builds for all these other devices to demonstrate how close to iPhone equivalent they were.
It's no accident that the Android golden age began with fusing the UI concepts of WebOS that worked with a rendering pipeline that started to make sense for the hardware it was on.
Mozilla ignored the autonomy of the parties that should be in an ecosystem and tried to make them wait for Mozilla's choices and implementations on things. I bet they had more than enough people on every one of their waiting lists for being involved but they discouraged actual involvement.
BBM was much more mass market than your attempt at revisionist history would suggest.
Unfortunately there's very few KaiOS phones on the market anymore. Nokia stopped making them and reverted back to Series 30 and 40. I wonder why.
Actually this is the reason that I, at the time working at Mozilla on the rendering team and helping with Firefox OS, proposed will-change. To solved this exact problem. With it, I managed to get the home screen to be perfectly smooth using pure web technologies and no hard coding on low end mobile hardware. Here's the original proposal: https://lists.w3.org/Archives/Public/www-style/2013Nov/0414....
FFOS was doomed because the pixel explosion wasn't just stretching the GPUs of that time, it had also made CPU rendering a serious bottleneck, a fact the Android team also struggled to accept. Mozilla lacked the resources to do anything about it, although I didn't get the impression they actually understood the problem at all (as shown by it not being mentioned here), while the Googlers resorted to that most Googly of activities: arguing via walls of text that try to deny the obvious truth.
I have long maintained that what Mozilla should have done was a Firefox OS based e-reader. The mania that exists for all things e-ink in the tech industry is real, and if you combined that with a proper open platform you could leverage it into being a self sustaining business.