But it's not just HTML and CSS - you need to create a completely new user journey because you can't make asynchronous requests to the server. In some instances you may even need to manage server requests completely differently. UI elements may have to be changed as well as you can no longer rely on Javascript to animate them.

You're essentially maintaining a second UI that is divorced from your main UI. There's significant cost in that. It's the same reason most sites don't support IE6 anymore.