I wouldn't call C# a super dominant language. it is extremely popular in the corporate world, where paying for something is seen as a good thing, because it means you've got support from a vendor.
In the start up culture, where technological innovation and taking risks are core values, people tend to favor maximum freedom and maximum technological possibilities, which are often in contradiction with what a corporation would like you to do.
Now that corporations want to have the same level of innovation you usually find in start ups, they start to want to use the same tools and embrace a bit of its culture. Note that Microsoft saw that and have announced massive open sourcing of every part of the .Net ecosystem just this year (which made a big impression here on HN, judging by the comments).
And now we've got Apple and swift, and people start to wonder if it's going to be "Ballmer" open sourcing, "Nadella" open sourcing, or "Stallman" open sourcing.
It's not just a matter of ideology. It will have practical consequences on the possibilities of using the language on every server configuration, of building innovative software with it, things even people at Apple wouldn't anticipate. Or if you're going to remain constrained to where Apple wants to allow this technology to grow.