This was my first thought too. And there are a lot of questions that will get asked, like, will all crate library names start being prefixed as well? So you end up with

use::bar; // changing to

use::foo::bar;

I assume the library names that can be overridden in cargo would still be accepted, and then it all gets a little messy. The transition would be very messy.

My preferred syntax route is a new separator in package names and the lib name gets populated by everything after it.

Still doesn't solve all of the policy problems with namespacing.