Xoogler here. I never worked on Fuchsia (or Android) but I knew a bunch of people who did and in other ways I was kinda adjacent to them and platforms in general.

Some have suggested Fuchsia was never intended to replace Android. That's either a much later pivot (after I left Google) or it's historical revisionism. It absolutely was intended to replace Android and a bunch of ex-Android people were involved with it from the start. The basic premise was:

1. Linux's driver situation for Android is fundamentally broken and (in the opinion of the Fuchsia team) cannot be fixed. Windows, for example, spent a lot of time on this issue to isolate issues within drivers to avoid kernel panics. Also, Microsoft created a relatively stable ABI for drivers. Linux doesn't do that. The process of upstreaming drivers is tedious and (IIRC) it often doesn't happen; and

2. (Again, in the opinion of the Fuchsia team) Android needed an ecosystem reset. I think this was a little more vague and, from what I could gather, meant different things to different people. But Android has a strange architecture. Certain parts are in the AOSP but an increasing amount was in what was then called Google Play Services. IIRC, an example was an SSL library. AOSP had one. Play had one.

Fuchsia, at least at the time, pretty much moved everything (including drivers) from kernel space into user space. More broadly. Fuchsia can be viewed in a similar way to, say, Plan9 and micro-kernel architectures as a whole. Some think this can work. Some people who are way more knowledgeable and experienced on OS design seem to be pretty vocal saying it can't because of the context-switching. You can find such treatises online.

In my opinion, Fuchsia always struck me as one of those greenfield vanity projects meant to keep very senior engineers. Put another way: it was a solution in search of a problem. You can argue the flaws in Android architecture are real but remember, Google doesn't control the hardware. At that time at least, it was Samsung. It probably still is. Samsung doesn't like being beholden to Google. They've tried (and failed) to create their own OS. Why would they abandon one ecosystem they don't control for another they don't control? If you can't answer that, then you shouldn't be investing billions (quite literally) into the project.

Stepping back a bit, Eric Schmidt when he was CEO seemed to hold the view that ChromeOS and Android could coexist. They could compete with one another. There was no need to "unify" them. So often, such efforts to unify different projects just lead to billions of dollars spent, years of stagnation and a product that is the lowest common denominator of the things it "unified". I personally thought it was smart not to bother but I also suspect at some point someone would because that's always what happens. Microsoft completely missed the mobile revolution by trying to unify everything under Windows OS. Apple were smart to leave iOS and MacOS separate.

The only fruit of this investment and a decade of effort by now is Nest devices. I believe they tried (and failed) to embed themselves with Chromecast

But I imagine a whole bunch of people got promoted and isn't that the real point?