You literally live in this world right now.
"It's already painful to compile and run Android from source. Fuschia will make it just impossible."
You can literally go download and compile the entire fuchsia kernel, right now.
How is that "impossible"?
Only the kernel Google puts out for the development image. I think the parent meant more in the sense of real devices. It is already quite painful to run custom Android builds from source in real devices, where at least the kernel has copyleft protections. It is quite likely that real hardware running Fuchsia will not come with their sources, since Fuchsia isn't copyleft.
Android discussions started that way. Things changed after enough time and revenue with a huge gap between ASOP and Android experience. Their security fixes vs Apple are also now abysmal. Might be a hint at the future of Google's next OS.
No, things changed for other vendors and for other parts. You can, AFAIK, still happily compile Google's entire kernels.
You are thinking of userspace.
Google's, sure. Some vendors make it painful (and in some cases, even impossible) to compile kernels for their devices.
Google is legally obligated to release kernels for their device. With Fuchsia, neither it nor any of the other hardware makers would be. Google might still continue to release their kernels — say, for developer contributions — but other vendors are quite likely to not do so.
Vendors won't modify fuchsia or its microkernel (magenta). That's the point behind the driver APIs in fuchsia. This should allow Google to update the full system, kernel included, while leaving the vendor drivers, which run in user mode and will still work due to the API being still supported, alone.
If Android is ported to fuchsia, that would solve the android update problem for good.
Is it really Android advertised to the West that people want if it doesn't have the userspace? That's like Windows open-sourcing the kernel but all the needed apps are proprietary. Might as well consider the overall thing proprietary unless your customers exclusively want the kernel plus also-ran software.