People like Optional and Stream because they allow chains of method calls. However, chaining only works for methods that are predefined. If an application wants to have a method or function that does something on a Stream (or Optional) and returns something that itself is chainable, the call to that method or function disrupts the nice linear chain.

For example, consider a method that transforms a stream by conditionally adding a filter stage:

    <T,U> Stream<U> maybeAddFilter(Stream<T> s) {
        if (condition)
            return s.filter(...);
        else
            return s;
    }

And suppose we have this pipeline:

    source.stream()
               .map(...)
               .collect(toList());

If we wanted to add the filter after the mapping operation, it would look like this:

    maybeAddFilter(
        source.stream()
                   .map(...))
        .collect(toList());

This disrupts the linear flow of the pipeline stages and makes the code confusing and hard to follow.

This could be mitigated by having a function called (for example) "chain" that takes a function to be chained, calls it passing "this", and returning its return value. On Stream<T>, this function would be declared something like this:

    <U> chain(Function<Stream<T>,U> func);

It would be used like so:

    source.stream()
               .map()
               .chain(this::maybeAddFilter)
               .collect(toList());

The chain method on Stream<T> wouldn't necessarily return another Stream; it could return anything.