Pixelmator team ?
I guess another way to look at it is this: If you had an idea for a desktop app today, would you decide to write for Mac first using a proprietary language Swift and proprietary SDKs like CoreData or would you pick C# and .NET so the bulk of your code could be re-used on both desktop and mobile, and you could bind to native toolkits for the UI?
Who are some big C# developers that create major applications consumers (Not enterprises) use everyday?
You mentioned the paucity of major OSX/iOS developers, but I'm not sure the case isn't the same for C#/.NET.