The pressure on margins for those that remain will come from those that control the full stack versus those that don't. A full stack developer can sell the device at near cost then recoup on app and content sales. The Nexus 7 and Kindle Fire are two examples of Android devices that follow this model.

I'm not sure what the margins are on the current Nexus phones are but if the next phone is of similar quality and has similar margins to the Nexus 7 then its going to put Samsung and HTC in a very uncomfortable position, especially if Google uses Motorola to build it. HTC is especially vulnerable as their operating profit last quarter was low.

Similarly if Amazon decides to do a Kindle Phone similar to the Fire that's going to put Samsung and HTC in another difficult position unless Google is going to start sharing more of the app and content 30% with the remaining phone vendors than they do today.

The key point is that the market has moved from devices and platforms to ecosystems. Unless you can derive revenue from all parts of the ecosystem then its going to be very difficult for companies that can't do that to compete with others that can.