I have project where I need to design & build a complex interface to sit on top of an ASP.NET/MVC4 back-end and run as a web-based application in a browser. The client has an existing "sister project" built using Sencha ExtJS and the GUI is a "recreation" of a Windows desktop, with floating, draggable windows, a "start" menu, tiling & z-indexing of windows, icons on the desktop, all running inside a single browser tab. In either IE6 or IE8 and probably on a 1280 x 1024 monitor (big corporate client with lazy IT department). They want the new site to mirror this existing "windows-style" version.
I think this is nuts: they're losing all the power of a real desktop environment (inherent speed & functionality such as alt-tab, keyboard shortcuts, system-drawn windows) and also not using the native functionality of the browser, such as tabs for separate "windows". It's almost like they're ditching the good things about each of those two environments and only using the bad things, and will end up with a clunky & unreliable app with massive impact on the user experience and totally going against the users' expectations of a website. (They are for the most part what you might call "low-level" users, with not much experience out of the MS Office "sphere").
I'm trying to construct a reasoned & balanced argument (that doesn't incorporate my perhaps obvious disdain) and I was wondering, other than my reasons above, is there anything obvious I'm missing (pros or cons) as I can't think of a single reason supporting building the app this way.