I hope Nikon does stay in the "High End" DX game. Canon abandoned their "Pro" DX body when they came out with the 1Dx announcement. You can be almost certain that you will never see a DX sensor in a D1-4 body either, but maybe there will be an improved D400(DX) in the Spring. Since upgrading my D300 to a D3, I've always thought they could have done better than even the D300s.
As for the D600 missing button, I do not miss it. I have never needed the AE-L/AF-L button and do not even use the Function or Preview buttons. I do however reprogram the AE-L/AF-L button to AF-ON and use that to focus each shot.