Valid points. A single colon would suffice, but I suspect in C++ they wanted something a bit more distinct from the semicolon. For Rust, I imagine at least part of the reasoning was keeping with what was familiar wit h C++, and after that the type names probably did make it complex, if not intractable (without reserving specific names).
Of all the glyphs used in the language, :: is probably in the top 5 for frequency of use (probably higher than even * in Rust, considering the infrequency of pointer usage and that multiplication isn't that common an operation in general code) but uses two glyphs instead of one.
It would have required different design constraints starting out to figure which glyph would be best suited instead of ::, but :: is still really ugly in all the languages that use it when its such a common symbol. I wouldn't argue overloading . is right, but I'm having a hard time seeing why a single colon doesn't work - foo:bar:baz. Maybe the parser finds scoping and explicit type names on definitions (ie let foo: i64 or in function definitions) require too many passes to distinguish?