That is your view.. .flatten() was explicitly requested instead of .flat_map(|x| x) on an RFC about identity and many wanted it.. The operation is also a fundamental operation as it is monadic join... It seems to me strange that bind and functor mapping should be there but not join...
My general view is that the standard library should provide a strong algorithmic foundation to build things upon and iterators are key here - I think there's room to grow in std::iter.
To reduce the cognitive load, we should better support signature based searching - a type of a function can very often tell you a lot about its semantics.. at least, that is what I've found with hoogle & Haskell.