If Swift means I can build a complete, releasable app for for iOS on Linux then I'll be all over it.
I work for FastMail. We have hybrid apps for Android and iOS (that is, apps that embed a webview and expose certain native features to the Javascript environment, which get detected and used by our regular mobile webapp.
I use Linux - its all I've ever known. I'm employed primarily to do operations, but I did our Android app because I use Android and it was interesting to me, and because the devtools are available for Linux I can mess with it from the couch in the evenings.
The iOS app was done by our main UX guy, who happens to be a Mac user. He did just enough to make it work, but doesn't have the time or interest to push it much further. I am interested, but I can't work on it from the couch. I have test devices available and they're easy to bring home. Bringing a whole Macbook home that doesn't have my normal tools and environment available; that's just a bit too much effort for stuff I mostly do on my own time.
As a result, the Android app is pretty good and getting lots of new features all the time. The iOS app meanwhile languishes. It works, but it doesn't have all the nice platform hooks you might like (eg sharing from other apps).
I wanted to work on an app for another platform in my spare time, mostly to test some architectural choices to make sure they work across all platforms. Based on devices we had in the office, I could choose Windows Phone or Blackberry. I chose Blackberry because it has development tools for Linux. At the time Windows Phone did not, though that might have changed with VS Code. Yes, I built a Blackberry app by choice (its actually really pleasant to program for).
If if turns out that Swift can build real apps for iOS from Linux, then I will be very quick to port the Objective-C parts of our app to Swift and its going to get a lot of features pretty quickly. That's going to make our customers happy. Many of them are Mac users. That can't be bad for Apple.
(I don't even care about the open-source bit here; just the availability of cross-platform tools).
Maybe part of this is Apple acknowledging that restricting development for their mobile devices to users of their desktop devices is actually limiting the number of apps for their platform? I can't imagine its a huge concern, but what does this lose them, really. Worst case, only the Mac users write apps for iOS and everything stays as it was.