It seems pretty clear they have 0 plans on doing that. They could've open sourced it in the first place, along with Combine.
Not open sourcing Combine seems like a pretty short sighted decision to me.
There's nothing stoping anyone from implementing such a thing, just as someone wrote a SwiftUI for HTML. All the pieces are there, but it would be a huge amount of work unrelated to SwiftUI on Mac and iOS.
1. Create and introduce it as proprietary
2. Iterate fast, changing a lot of things to improve it quickly.
3. Stablize
4. Open source it (and continue iterating with an open process)
The advantage to starting proprietary is the initial iteration phase can move move quickly and stay focused on the goals of the people in the controlling group.
Dr. Dobb's had an article in 2014 [2] which said "Swift is proprietary and closed: It is entirely controlled by Apple and there is no open source implementation."
EDIT: I found a HN post from 2015, just over a year after Swift was released, titled "Swift will be open source later this year" [3]. Not immediate at all.
[1]: https://asciiwwdc.com/2014/sessions/101 [2]: http://www.drdobbs.com/architecture-and-design/swift-objecti... [3]: https://news.ycombinator.com/item?id=9680982