IE11 only works on Windows 7/8, and OS X 10.10 is only supported by Macs up to about 5 years old. Even then, a lot of people don't actually keep their OS/browsers up to date. I expect Netflix's demographics are quite broad, and probably still quite a lot of people on XP and 5+ year old computers. There's not really any reason for them to drop support for Silverlight, they'll probably just keep new features for the HTML5 player.