Well, since the game is no longer married to Steam at that point, my users would be losing Steam's achievements, mods and matchmaking. Am I not understanding that right?

They showed off installing mods from Steamworks in Age of Empires II during the demo.

Those were mods from SteamWorks? Sorry, I didn't catch that part. I still genuinely wonder why I would want to convert my app into a "modern desktop app".

    Sandboxed security. 
    Easier install/uninstall.
    Backup/restore.
    Live tiles and notifications
    Downloadable from the Windows Store (which means it will show up in Cortana search results when searching for it, even if it's not installed)
    Ability to use excellent new WinRT API's
    Deep linking
    Share to other apps easily (e.g., post to Facebook)

etc.

Thanks.

To make is discoverable by non steam users?