These are interesting times for Windows developers, with the PC in decline and Microsoft's mobile platform suffering from tiny market share.
The home PC may be in decline, but most C# developers are paid professionals who work for businesses where the PC is in anything but decline. I'm comfortable writing Java code, so have little to fear from the heat death of MS, but being realistic, barring "Linux on the unicorn" ever coming to fruition, the desktop PC will be around for decades, as will Windows and C#.
Java, on the other hand, is fast becoming a dead language, outside of big data and Android circles. Look at the 5 year plans for each language stack - MS are being very ambitious whereas Oracle, well, just aren't.
The mobile market I'll give you, hence my interest in Java & Android.