I'm unfamiliar with Swift but Objective-C is an excellent language. I much prefer it to C++.

Cocoa won't be made Free, no, but perhaps Swift could be used with GnuStep's implementation of the libraries.

It's better to not re-invent the wheel.

Yeah. C++ just got too ornate. The "just bolt on objects" to C in "Objective C" ( though a complete hack ) was all that was "needed".

Swift could be a cross-platform option for Android and Windows mobile. There will soon be hundreds of open source Swift libraries. Swift could also be an alternative to Java, which has large 3rd party library support.

The language itself is lovely. What parts of cocoa feel essential to you?

Actually, if you want to do backends, nothing is essential from Cocoa itself.

I think Foundation might be necessary though, even if you just use Swift structs (Array, Dictionary, etc)

This. I'm SUPER excited to have a Swift backend for a web app, and/or I've been toying around with the core logic of another app. Either idea doesn't need anything in the way of GUI, just the standard library and maybe a few things from Foundation that should be in the standard library anyway (like pseudorandom number generation).

"Essential" ? Any of the objects that can give a "dashboard" interface and maybe draw a picture.

I'd love a portable GUI API that satisfies some harsh conditions: 1) Portable between mac,linux,windows,android,ios. [ must re-compile to target ] 2) Compilable into a single static binary. 3) New developments don't break backwards compatibility.