I don't think Android is a good platform for desktop usage. First the windowing system, and the IPC mechanism. They are very limited. And one of the nice aspects of desktop computing is the ability to alter it for your own purposes (something that MacOS is running away from). Meaning you extend it for some other domain, think music production, video production,... Where you want to hook it to some hardware and have the software to talk directly to the latter. Which means having access to all the ports and coding bespoke protocols. I don't think current android API allows for that.
> The distros could add new APIs could be added to expose new capabilities.
Those exist already. With Debian, Alpine, Fedora,... you can put anything on top of the kernel in the userland. Android goes with a restricted version.
It's the same with MacOS. It's Unix, but with proprietary add-ons and systems. And lately, with more restrictions.
How does those existing make Android not a good platform? I don't fully understand the point you are trying to make.
By restrictions do you mean having proper capability based security instead of letting all apps have access to everything? These restrictions are a good thing.
Limited userland and limited access to it. Sandboxing may be good, but the user may need some software that needs to escape it. Fedora Silverblue is a promising direction, but interacting with the base system is currently a pain.
