Project Treble didn't fix anything (I'm very happy with Project Treble, because it lets me hack stuff I couldn't begin to imagine with 4 yrs ago, but that's just hack). It didn't even fix Nexus/Pixels that still have the same upgrade track record as a Samsung 100$ device. (Pixel 1 and Pixel 2 users, feel free to come and use my Generic System Image, I'll keep upgrading you as long as I can with my bare hands!)

ATM OEMs are NOT allowed to use Project Treble. There is only one OEM in the whole world that did an upgrade using Project Treble. What does this OEM have special? Well it's Meizu, they ship only in China, and doesn't care about Google certification. The certification that you say helped prevent Android from being fragmented. Turns out it actually helps fragmentation.

Meizu, Amazon, and Huawei are not Google certified. I have yet to hear developers say they are specifically worse than other Android devices. Last time I had an Android device with stupidly broken video aspect ratio (I'm a multimedia developer amongst other things), it wasn't a Meizu, Amazon or Huawei device. It was a properly certified Philips TV.

Project Treble is sincerely a superb engineering project. But the truth severely lags behind.

If you don't care about Google certification, you can upgrade an Android 8 device to Android 11 finger in the nose, keeping Android 8. It's bootable in a day, usable in a week, production ready in a month. If you care about Google certification? Well, you need to upgrades your drivers to Android 11 base, which is a non-sense when mentioning Project Treble.

How to upgrade drivers? Well it depends. Usually the first step is to upgrade your build scripts. Because well of course, every two major upgrades Android build system is "upgraded". Well, OEM got the trick, now they are building their drivers out of Android, and their build system is a giant shell script. Way to go! Makes upgrading to newer Android version much better!

Now you've managed to build your Oreo drivers into Android 11, congrats! Now let's start them! Oh, new "security" features have appeared, and now half of your code no longer works. (fdsan I believe is way more annoying that it is meant to be)

Let's not forget about Android 9 Go mess as well. (Hint: The only post-Oreo Samsung devices that didn't get their 2 upgrades are Android Go devices) Basically Google cares about RAM consumption two weeks every 2 year, so only this exact release is usable on low-end devices.

Is Google evil? I don't believe it is. Hell I believe that every Google engineers believe they are working towards a better future. However I do believe that even within Android itself, there are many units, that are (mostly unwillingly) shooting at each other's foot. So yes having AOSP is awesome, Project Treble is beyond any hope I had back then. Still, overall the improvement is far from what has been advertised.

To come back to Fuchsia, I believe it is going to be pushed to Android devices in two years (so Android T). OEMs won't have a word to say in this: Google gave a try at such a big change through GKI, and OEMs have been complying nicely, so going to Fuchsia should be pretty easy too. I'm expecting some Android features to be offloaded to Fuchsia for "security" reasons, like Window Manager, which will constraint OEMs even further down as to what they can change, because the Fuchsia "core" image will be like Google Services, mandatory Google-signed. (There'll be Fuchsia app images as well, which people will be able to do whatever they want with, but that's something else) I hope I'm wrong.