Rebellion has all the stuff the others had. Once all the mods switch over there will be little reason to keep playing them. And why would steam just let Stardock do that if they didn't make any money off the old customers.
lol Why? Because every person that uses Steam is another person that sees their ads pop up for games/specials, which makes more game devs want to pay Steam to advertise, which makes Steam more money. Not to mention, as far as Valve is concerned, every person using Steam is a potential sell, as they will be more exposed to their store, etc...
Business, son! Business!