Look at what happened to the 580 EXII. New ones are almost impossible to find and the price has gone up well above what it was before the announcement of the 600 EX.
In the new market, yes, in the used market, not so much. That's one of things I think a lot of people are forgetting, there are a TON of 5dII's that were bought in the last year, and those people would probably upgrade to a better AF system if possible. There's also still the refurbished route, which Canon maintains on products they've discontinued like the T1i for example. I can get a refurb 580EX II for $339 +tax right now, and its $399+tax normally. I can't imagine the 5dII refurb disappearing from the Canon store, so, at worst, that price will stay the same
I don't think the price can go any lower. If Canon launches a 2K FF camera, I'd assume they would immediately discontinue the 5DII. The result could then raise the 5DII's price from smaller dealers. Just look what happened to the 24-70L as soon as it was discontinued!
And that is because there is no replacement. Simple supply and demand; a lot of people need the 24-70 in their kit, and its impossible to get a new one from an authorized dealer. If Canon completely killed off the 5dII (0 stock anywhere, no refurbs) and didn't deliver the 6D for a year, then yes, 5dII prices would go higher. But they'd all be used, and it'd be a different story. It'd be a secondary concern to the fact they'd have destroyed their reputation and lost a lot of photographers who bought Nikon instead
Also, note that lenses hold their value much better, often rising in price over time. The same isn't true of camera bodies, where you are lucky to hold 50% of its value 5 years after its released.