You'll see people that all ready write .NET keep writing .NET though :)
The ability to rent a cheap VM to run a C# MVC application is great! Previously, I only wrote C# @ work, now I have the ability to do it outside and in. No one wants to pay XX$ more per month to license windows on a VM(at least not for the hobby stuff I do outside of work for fun)
I imagine it will be the same for iOS developers, it may not draw in new developers but there is a good chance that is may be used on projects that would've typically used a different platform simply because swift only runs on iOS.
The language probably won't see an explosive growth in new developers but you will see swift being picked over another language in places it wouldn't previously run.
It had significant impact on the game industry. With Unity adopting .NET as its scripting / game logic layer, quite a few games are actually based on .NET code (including the ever-popular rocket-science-in-a-box https://kerbalspaceprogram.com/).
