>We have VERY long history using file system, it is the base of every OS, it has solid API, every language provide similar access interface, everybody familiar with how to talk with file system.
I agree, but then you don't really do that, do you? Merely mimicking the Rust fs API is not enough to call your project a "filesystem" in my opinion. It's actually probably the least of my concerns when it comes to using a filesystem.
On the other hand not being able to use the familiar "cd", "grep", "cp", "mv" and "cat" the contents of this so-called filesystem means that it's really not that familiar at all.
I'm not sure I see the point of the "minimal attack surface", you can create private fuse mounts that can't be accessed by other UIDs. And if you're worried about same-UID compromised programs snooping around then it's game over anyway. That smells a bit like FUD to me.