I don't know how many copies Thief or Thief 2 sold, but I'm sure it wasn't enough to break even. Of course they're going to make changes to make the game appeal to a broader base because there aren't enough "hardcore" fans to make it worth their while otherwise. Besides, 80 percent of the people who really enjoyed the first two will buy the game eventually, no matter how much they whine about it. It makes sense.
Besides, I don't see why everyone is complaining so much. The game will be good enough. Here's an idea to all the whiners: if you're not utterly astounded by the game, FIND SOMETHING ELSE TO DO!