In theory, this could happen. In practice, Ruby/Python/JavaScript seem to be doing just fine.
I assume Rust will be around for a while (I certainly hope so - I've just started learning it!) so it might be wise to plan for growth now before it's too late to implement.
While it's true that sometimes collisions happen, in practice, you just choose a new name. Everything is significantly simpler, which in software is very often a Good Thing.
Just from my own experience, I know that it makes me less likely to make packages public if I have to give them odd, collision-avoiding names first - especially as I then have to use those names myself. It's particularly annoying when the incumbent package is 10 lines of poorly-written abandonware that hasn't been touched in years, which often seems to be the case.
I know this isn't an issue for Rust now but I'm thinking about ten years down the road when you have 250,000 crates on the site, 99.9% of them junk, clogging up that flat namespace. (Python's repository has more than 50,000 packages and seems to be growing at an accelerating rate now that it finally ships with a decent package installer by default.)
Nothing is impossible, of course, but the language already has a flat crate namespace. So you could either implement some kind of layer on top of that, or change the language itself to have some sort of crate hierarchy somehow. All just to have two crates named 'http' rather than one named 'http' and one named 'requests' (for example.)
Here's another way of looking at it: shipping a flat namespace is a proven, well-worn solution. Adding in a hierarchical one, on top of the language, which has a flat one, provides dubious benefit, for significant implementation complexity.
Seriously, I do get that there are trade-offs. I'm just interested in how you've been weighing up the alternatives as I do often find myself wishing that other projects I've been involved in had taken the hierarchical approach at the beginning when it was an option.
Developers who really like a name could always add something such as using "moz-foo" instead of just "foo".
I've been surprised with how few name collision problems I've had on Python's Pypi. If "z-foo" module is more usable than an unmaintained "foo" module, everybody recommends and uses "z-foo" without a second thought.
https://www.npmjs.org/doc/misc/npm-disputes.html
Not an urgent need, but a similar process will need to be implemented.
Naming conventions could be nice too.
Naming conventions are currently "the same as Rust identifiers".
I'm somewhat uncomfortable with the idea, but I'll gladly defer to people with more experience on this matter than me.
Besides, the only alternative I can come up with, is some form of procedure for voting out crates which doesn't play nice with the community...