The 400 f/2.8 is the all time favourite with sports shooters, along with the equally remarkable 300 f/2.8. Even older beat up 400 lenses hold their value remarkably well. I think you'll see a lot of demand for 400 f/2.8 lenses as the 1DX starts to sell in meaningful numbers to sports shooters who are upgrading from MkIV bodies with their very useful APS-H x1.3 crop sensors. This will keep second hand prices buoyant for some time, even as photographers upgrade to the 400 f/2.8II and release their old lenses onto the secondhand market.
With the FF 1DX shooters who rely on the reach of the 300 f/2.8 and APS-H will be wanting more, and their answer lies with the 400 f/2.8.