As I said in a parallel comment, there is a fairly large difference between archiving your own project's history for as long as you feel like, and archiving the complete history of every significant piece of code ever written in a particular programming language forever.
Kernel.org's repository is also of major versions, not every minor release and patch. That really wouldn't do for cargo. If it has ever been released, it needs to be kept in storage for as long as the rust ecosystem exists. That's decades, maybe even centuries of passing on the torch and hoping the next guy accepts the responsibility. Hoping you can find a next guy.
Kernel.org's repository is also of major versions, not every minor release and patch. That really wouldn't do for cargo. If it has ever been released, it needs to be kept in storage for as long as the rust ecosystem exists. That's decades, maybe even centuries of passing on the torch and hoping the next guy accepts the responsibility. Hoping you can find a next guy.