One doesn't rule out the other. You can build HTML5 apps for Windows and use the same technology that you would use for the browser version.
Sky's insistence on continuing to build their player on legacy tools and their refusal to properly support Windows, which by the way still provides the largest userbase by some margin, is incredible. Yes, moving to HTML5 and supporting a platform costs money and requires resources, but considering the amount of money they charge, this is no excuse. At all. Many other far smaller organisations with less resources manage to do it just fine.
You would think that at this stage where all the major browsers par one (and one legacy browswer) no longer support Silverlight Sky would get the hint. Have they? It doesn't look like it.