The same printer works with Linux the same it has always done. But that's not Windows' fault or to Linux's credit. It's just the result of a really crappy vendor. However the Linux API is generally more stable than people give it credit for, it's just that it's the C headers only. Anything compiled for a kernel generally won't work anywhere else, and that's not what the crappy vendors wants.

In the embedded space, including most mobile phone vendors which seemed to be an important use case for Fuchsia, even reputable vendors will generally give you an image of an operating system heavily modified to work with their hardware. That's their "driver". Imagine buying a new PC and receiving a DVD with a Windows modified to work on that hardware only. Of course you can't upgrade or even patch security issues beyond what the vendor will give you! You're supposed to buy new hardware. Sure, you could extract the drivers and try to install them on a vanilla Windows, and that's exactly what projects like LineageOS do, but most users won't bother.

That's the situation with phones. It's not at all clear why Fuchsia thinks they could solve this. It's a cultural and an economical problem and can't be fixed with software alone. Why would phone manufacturer care about your microkernel architecture? They will just patch the whole operating system, binaries and everything, until it boots enough to start the GUI and ship that. Just like they always have.

The only thing that could improve this situation is by enforcing the GPL, or have similar contractual stipulations like only being allowed to ship a reference implementation unmodified, but Google shows no interest in doing that. They care about getting Android on as many devices as possible with no regard to their respective quality or product longevity.