How do I, at a given point in time, find the best platform for doing somewhat future-proof GUI?
INMO, this is a desirable goal but neither realistic or well defined.
It is not well defined because you really should quantify the 'future' part. It is not realistic because history tells us that GUI technologies are the fastest to change, and their changes are radical.
Back to your question, one needs to see why this is important in real life applications. You see, even older technologies are not always dropped suddenly and the live for a long time.
Control vendors are still providing upgrades to Windows Forms application GUI components. You can develop a touch-based application today using Windows Forms. Something you probably can't do with native out-of-the box newer Silverlight!
The first thing I would do is to decide on your expected user environment. Next, I would identify the types of devices that the application must run on (applets, mobiles, web, desktop, etc.), then you check the supporting languages and tools.
WPF and Silverlight will still live, though may not be the magnet for improvement dollars. XAML, in some form, will be used in the Windows 8 world. if you think along the Silverlight path, then check the System Requirements matrix on MS-Silverlight.
Developing an application that is not dependent on the GUI, does not sound very realistic for many commercial business applications at least. The extra complexity is very difficult to justify. What business people sometimes tend to find is that the technology outlives the business model some applications are built on. (I remember a project we did for a large bank that lived for 3 months only because another bank acquired the one we did the project for :)).