I have no idea what ART is but I doubt it addresses all concerns I listed above. On Android, there are plenty of native libraries and apps these days as well. I'm pretty sure these don't work as is without a compatibility layer that essentially replicates a lot of linux/posix stuff, which Fuchsia does not implement.
In any case, it wouldn't be the first time that Google walks away after putting lots of development in something. In general, I think they are closer to merging Android and Chrome OS than they are to replacing either with Fuchsia (not to mention convincing OEMs like Samsung to actually use it).
Since Android 7, Google has been claping down NDK users that try to used anything that isn't part of that list.
Since Android 8, APKs are only allowed to reach for their own internal filesystem and use SAF for anything else. Something that will be further enforced on Android Q, so no luck trying to peek into /dev, /share and similar.
In any case, it wouldn't be the first time that Google walks away after putting lots of development in something. In general, I think they are closer to merging Android and Chrome OS than they are to replacing either with Fuchsia (not to mention convincing OEMs like Samsung to actually use it).