As far as I could tell, its main goal was to have fun writing an OS. At that, it seems to have succeeded for a number of the people involved?

In terms of impact or business case, I'm missing what the end goal for the company or execs involved is. It's not re-writing user-space components of AOSP, because that's all Java or Kotlin. Maybe it's a super-longterm super-expensive effort to replace Linux underlying Android with Fuchia? Or for ChromeOS? Again, seems like a weird motivation to justify such a huge investment in both the team building it and a later migration effort to use it. But what else?