Yeah, but you're not even really writing a web app. You're writing a Silverlight app whose VM happens to exist inside a web browser. Based on what you've said before, the exact same thing could just as easily run outside the web browser, and even be launched from the browser itself. And if it can run inside the browser, the software already exists on the machine to run it completely independently of the browser. I suppose that at least starting it from the web browser gives you some control over version management.

So what you're saying is that your customer wants a desktop app, but they also want it, for no good reason, surrounded by the trappings of a web browser window.

I'm not saying you can do anything about that. I'm just pointing out the absurdity.