One case where namespaces may not be so useful for achieving "static classes" is when using these classes to achieve composition over inheritance. Namespaces cannot be friends of classes and so cannot access private members of a class.
class Class {
public:
void foo() { Static::bar(*this); }
private:
int member{0};
friend class Static;
};
class Static {
public:
template <typename T>
static void bar(T& t) {
t.member = 1;
}
};