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.
Who claims that archiving the complete history of every significant piece of code ever written in Rust is necessary? It is easy to archive only the code that your project depends upon. Rust code is no different from C code in this regard.
- crates.io is financed by the Rust Foundation and is at no risk of disappearing, it is a very well funded effort.
- Using cargo with an alternative repo is not difficult, requires some one-time configuration.
- Vendoring your dependencies is supported.
- cargo hits the network to look for semver compatible updated versions of your dependencies on specific moments if you don't have a Cargo.lock file.
- Not updating your dependencies stops you from getting the rug pulled from under you if an unwanted change happens, but it also stops you from getting any desired changes including security vulnerability fixes.
- Even if you vendor all of your dependencies, you still have to audit them the first time and every time you update them. Are you? Most aren't. Code you haven't written yourself can't be assured not to be malicious, and code you've written yourself can still have exploitable mistakes.