There is already a very capable visual XAML Designer included in Visual Studio .NET so this isn't a problem. Copilot says:

Key Features:

- Drag and Drop: You can drag controls from the Toolbox and drop them onto the design surface.

- Properties Window: Allows you to set properties for the selected control, such as size, color, and layout.

- Split View: You can view the design surface and the XAML code side by side. Changes made in the designer are reflected in the XAML code, and vice versa.

- IntelliSense: Provides code completion and suggestions while editing XAML code.
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The real issue is that people already using .NET to create the UI would be crazy to go back and use it in VB6! (well except Franky of course)