Does it really matter ? While I think Swift is an amazing language, it would be much less useful outside the closed ecosystem of Apple. The same thing happened to C#, it never really took off outside Windows.
e.g.
- Xamarin (multiplatform mobile apps)
- Unity3d (multiplatform games)
- Monogame (multiplatform games)
- Unreal Engine 4 (build system)
I guess Swift could fill the same gaps. I would especially love to be able to develop multiplatform mobile apps with swift since switching between ObjC and Java all the time is quite taxing.
1) It did take off like wildfire on Linux as Mono
2) People claimed Mono was a trap and not to use it. Because you can never trust Micro$oft
3) personally sad this happened to an open standard language
There are several massive communities that use Monogame (even still use XNA) and Unity. It's huge in the indie community, and is basically how the XB1, PS4, and Vita expect you to make games for their systems. The Vita Tool Chain is mostly C#.
Sure, no one is writing AAA games that require maximum graphical capabilities in C#, those teams use C++ exclusively (often with a scripting language on top). At that point that is really the only language AAA games use if they are made from scratch, otherwise they are using an engine already made in C++.
Bastion, Magika, A.R.E.S., Dust, Fez, Rogue Legacy, Reus, and Terraria are just a few made in XNA/Mono. And looking at total sales amounts, some of those games definitely had revenues over a million dollars.
That's not including unity games like: Shadowrun Returns, Rust, Wasteland 2, and Hearth Stone. These games made very good amounts of money, again, not 100% AAA, but still industry veterans.
It feels like one of those big business vs small business issues. If you have the man power and the resources, you can go much farther much faster. But not every starting company can afford the overhead of going full C++ and Engine heavy.
So C# being 'really' popular in the AAA side of the industry is a tad misleading (though can be used as a language on top of the engine, for tools (where it is probably most used), and for tool-chain), to discount it only due to the top 33% of companies usage, is a bit unfair to the quite large and growing indie and AA crowd.