I'm not sure why that has to be the case. For example, C# found a home outside of the Microsoft ecosystem inside of the Unity game SDK. That was a result of the initial grass-roots Mono [1] project. There was the same handwringing when Mono started and as it started to gain traction. Nowadays I think most people consider it to be a viable cross-platform language. I see no reason Swift can't go this way as well.

1. https://en.wikipedia.org/wiki/Mono_(software)