> People would start buying Steam Machines then replace the OS or even resell the parts.

That would be highly unprofitable. A subsidized Steam Machine contains a 7600M equivalent. It'll probably have a great price for machine with a 7600M, but it'll be significantly more expensive than a machine with an iGPU. Non-gamers aren't going to pay extra for a machine with a 7600M. And gamers are likely buying Steam games even if they aren't using SteamOS. You can't rip out the 7600M to sell it.