"unsound" has a particular definition that is separate from what people have issue with here; in Rust parlance, there's nothing unsound about the `as` operator choosing to saturate or overflow etc., because that has no ability to break the guarantees provided by the type system.
As for requiring an additional speedbump (like e.g. a "lossy" label) here to guard against misuse, I think this proposal is overlooking something: Rust can't just abruptly break all code that currently uses `as` in order to demand that something like `lossy as` be used instead. Any removal would have to first have a very long period where `lossy as` is syntactically valid and where the compiler instead warns for people using raw `as`. But if the compiler is already emitting a mere warning for `as` that suggests a better alternative, then it could just as easily suggest a method like `.try_into()`, which exists today. And once you're having the compiler warn about changing `as` into something else, that's already indistinguishable from deprecating `as` in those instances, so there's no point trying to avoid it.
