Nix being an expression based functional language, it doesn’t really make sense to have something like let x=y; since this looks to most people like a statement

I know of several languages that just have let x=y; <expr> as an expression. It works fine, it's just more obvious syntax.

The fact that it looks like a statement is kind of the point.

it follows the traditions of other functional languages such as the ML

OCaml at least supports both forms. They didn't have to pick the weird one.