I don't think having a great liberally-licenced microkernel-based OS is as good for the world in a practical sense compared to forcing every Android and Chromebook vendor to publish the changes they made to a copyleft monolithic kernel. I can totally understand being worried that the rise of Fuchsia would result in a dark age for running alternative open source OSes on cheap consumer smartphones, SBCs and ARM64 laptops.
Vendors don’t publish source code now lol
Some do. If the current state of things is what happens when vendors are required to publish their source by law, imagine how much worse it would be if they weren't - there'd be nothing. What little we have of FOSS OSes on cheap consumer devices would be gone. Imagine no LineageOS, no community-supported postmarketOS devices, no installing mainline Linux on old Chromebooks etc.
This is true in some abstract sense but the value really goes up dramatically if more than a few people actually use it. There’s a strong feedback loop around having drivers or being able to run common code, and that’s why many people were hoping that Google had a strategy since there aren’t that many projects with the resources to push through the early iterations of those problems.