The question isn't "Wither WPF?" or "Will Silverlight marginalize WPF?"

With announcements at MIX09 that Silverlight will gain many of the key features that made WPF special, developers, component vendors and the media are now concerned more than ever about the future of WPF.

It's not WPF they should be worried about.

When you elect to use Silverlight, you are really choosing to forego the full .NET framework and everything that goes along with it that isn't in the latest flavour of Silverlight. Tons of useful capabilities, thousands of APIs, file system access, you name it. And also lots of new .NET 4.0 capabilities as well, like dynamic languages, MEF, parallel programming, hard-core multi-touch support, and so on, that aren't in Silverlight.

Silverlight isn't just a compact, streamlined version of WPF, it is a compact, streamlined version of the entire .NET framework. To say the debate is about whether or not you want to forego some particular WPF features that aren't in Silverlight 3 is focusing on too fine a point. The issue at hand, and the choice being made when selecting the platform to use, is much larger than that.

Think about this question instead: Do you think there will be a significant amount of developers that will continue to use the full .NET framework for their projects? Or will they be using the more cross-platform, quick-to-install Silverlight, whether they are targeting the web or not? Consider both run-of-the-mill business apps which Silverlight 3 tries to easily enable, as well as other kinds of apps, such as “media” apps like movie players and games.

If your answer is that, Yes, there will still be many developers using the full .NET framework instead of Silverlight, then you have nothing to worry about when it comes to WPF. WPF is arguably at the point where it knocks the socks off of Windows Forms. With .NET 4, Visual Studio 2010 and Expression Blend 3, WPF will be the UI framework "par excellence" for those using the full .NET framework.

If your answer is No, you think very few developers will resist Silverlight's temptations such as being able to run on OS X and potentially other platforms, then it isn't just WPF that you should be concerned about. It's the entire .NET framework, even the Windows platform itself.

After all, if there weren't enough reasons left to develop specifically for Windows, well, all that would be left going for Windows would be features. There would be fewer and fewer Windows-only apps. That seems suicidal to me, Microsoft wouldn’t let it happen without a fight. What did Steve Jobs recently say? He insists it’s the apps that will differentiate Apple’s latest product, the iPhone, from the competition.

So what’s my answer?

Yes, moving forward, there will still be plenty of developers that choose to use the full .NET framework, for not-so-run-of-the-mill business apps as well as for a million and one other kinds of apps. I'm talking about a pretty big 'Long Tail'. And many of those apps will need a UI, and that means WPF, not Silverlight. Sure, Silverlight 3 has multi-platinum superstar potential, but WPF should get a few gold records as well. I think it’s far from withering away or being marginalized, as many are worried about.