AOSP absolutely is a massive success. The fact that all of these Android handset manufacturers exist today, is thanks to AOSP. The fact that Android is a major competitor to the Apple ecosystem, is thanks to AOSP.
I think the biggest criticism we can say is AOSP has not taken off like Linux with different forks being maintained and parallel, mostly because people who are using AOSP want to sell devices and not maintain an operating system.... If AOSP was LGPL, and all of the manufacturers who fork it, had to release their code openly, that would be interesting... Heck, if Google required open bootloaders, and the ability to secure bootloaders, to get Google Play services on the phone. We would see a massive amount of user empowerment in the entire ecosystem....
I think the biggest excitement we see for companies to fork AOSP is when they want to try to make their own walled garden like Google Play, like the app store.... So we see this with Samsung, the Chinese phones etc.
As long as Google does the heavy lifting of AOSP I don't think we're going to see major commercial forks, nobody got rich duplicating Google's work they give away for free.
I am very thankful that AOSP exists so we can get programs like replicant, and graphene OS. And will I love these projects, if AOSP stop getting updates tomorrow, there is not enough momentum for these projects to survive on their own, so they're more like very enthusiastic skins that depend on AOSP updates rather than independent operating systems.