The backwards compatibility argument is interesting. I think it's really compelling, but it's also worth taking into account the pains that the Haskell community has been feeling since standardizing the standard library as insufficiently generic.
I have no idea if a similar problem is possible with Rust, but it'll be interesting to see how HKTs enter the scene if and when they do.
The Rust standard library will exist as an separate entity from the language itself, and can be versioned, updated, and deprecated just as any other library can, without being tied to any specific version of the language.
However, the Rust team is trying to get 1.0 out the door, and higher-kinded types would be backwards compatible, so I don't think we'll be seeing them for a while. There's also questions about the overhead and syntax that would need to be resolved for them to make it in.