But for those that are used to Flash (specifically designers), how to convince them to move to WPF? Especially for Web Apps? Which Flash doesn't have to wait for plug ins, like WPF?
You really can't. Whatever advantages WPF brings to the table, is WAY offset by the disadvantages. Vista only, and BY FAR the change in workflow. 2D graphics & animation is what Flash does pretty well, and you're just not going to be able to change those minds.