Namespaces are a far cry from the capability security model that Wasm uses.
Yes, and Wasm's capability based security model makes sense for adding plugins to your app. But TFA is intended for running entire apps, so I think letting the user of the app control fs and network access via the already widely supported and proven method of CGroups and namespaces is a much better option than whatever method each individual app comes up with.
That's only true if there are no bugs in the kernel or anything that implements the namespaces. You're needlessly adding a huge attack surface.
Devil's advocate, using WASI assume there are no bugs in the new specifications or anything that implements those specifications. We already have a variety of tools to sandbox processes, by implementing and adopting WASI, you're needlessly adding a huge attack surface.