That makes no sense. It's pretty clear that .NET is Microsoft's future, and Xamarin extends the reach of .NET.
Look at Winforms, WPF and Silverlight. They are all .Net and all abandoned. I wouldn't bet my long term future on Xamarin.
Yes, Silverlight has been abandoned.
I would argue that Winforms is now a mature technology and requires little upkeep, but it is still a part of the stack.
WPF development has slowed significantly, but still proceeds.
None of these are good technologies if you want to stay up-to-date. You could also write your site in IE6 compatible HTML which was quite mature but that seems a really bad idea.
That's not a reasonable comparison at all. Winforms is maintained. IE6 is explicitly deprecated.
And WinForms is never really going to go away as long as Windows is still built around the Win32 API.
Silverlight is abandoned, but Winforms (especially) and WPF are mature products; I use the latter two (as well as Xamarin for Mac) in my consulting business to make good profits each year, and don't see anything changing except for better in this respect.
I would call Winforms and WPF "mature and maintained" rather than abandoned. Especially if compared to Silverlight.
I can provide a similar list for any big corporation, including Apple and Google.