It's like buying a car... depreciation starts in 30 seconds or 30 feet, whichever comes first, with cameras and cars.
5D and 5DII will likely hold value a bit better since they are sort of a cult camera and very highly regarded. But there will be depreciation.
On the bright side, the full framers have about a 3 year market cycle, between new model intros. That's better than entry level cameras with 12 or 18 months. Or G-series compact digitals that see a new model intro every year practically like clock work.
5DIII has long been rumored and is actually overdue... if the three years between the first and second models can be considered enough of a pattern to use for predictions.
On the other hand, both previous 5D models were announced about a year after respective 1Ds models - probably deliberately so the sale of the lower priced model doesn't have too much impact on the sale of the top of the line model - and we haven't heard word one about a 1Ds Mark IV yet, either. Maybe Canon will do that again. Maybe they won't. Who knows? Certainly not any of Canon's retailers... they are just guessing, like us.
There's also been a rumor that there won't be a 1Ds Mark IV, per se... That the 1D series might go in a completely new direction... With one model (maybe a 1D Mark V) that offers FF, 1.3X and perhaps even 1.6X all in one camera, can be fitted with crop sensor lenses, that has frame rates corresponding to the different formats, etc. Sounds logical, since other camera manufacturers that start with "N" have already done it in some of their models. But, who knows!?
Your camera store guy is really just speculating and spreading rumors.
One thing for sure, if you go buy a 5DII today don't worry, it won't stop taking great photos the day a new model is rolled out. If you're thinking investment, then you probably should look at gold or Apple stock, maybe Tesla if you have really high risk tolerance... instead of DSLRs (or a new car). If you are wanting to take photos, then the 5DII is still and will be a great camera for quite a while in the future.