You need those for memory safety. It is very true that you don't need Rust's memory safety features, such as Arc and Cell, if you don't want memory safety, but it's also not a very interesting observation.
There are alternative mechanisms that do not use Cell but still provide safe mutation of pointer-free data. Cyclone had one, and IIRC Rust itself even had one before a type system simplification.
Sure, and I'd be interested in exploring how those might be integrated into Rust in the future. Carving out subsets of pointer-free data seems pretty powerful.
(It doesn't seem like the language in the OP is interested in memory safety though.)
