I just don't think your statement holds true with such technology. Never will a £10K spend on a PC ever be a good long term/future proof investment. If Intel's processor upgrades follow as thy have done since Apple went Intel then history shows, this years top end model will be virtually matched in speed by next years low end model.
Photography software puts such a low drain on the resources of modern multi core machines that you'd be much better of buying the lower spec MacPro's on a more regular basis, than spending on the highest spec model and keeping it for 10 years. People who demand the latest and fastest from their computers (video, 3D, animation etc, etc) then fine but for photographers.
While certainly technology will continue to move ahead, I know my previous 8 core MacPro was from 3 generations ago, and was still very serviceable after 3 years (and in fact is in daily use still today, just not by me). I'm not talking about 10 years here, but maybe around 3.
So even though technology will continue to improve, those improvements are incremental, and upgrading every iteration of MacPro really doesn't do what it used to. Since PS is traditionally on about an 18 month upgrade cycle, it's conceivable to me that the current 6 and 12 core will still be quite useful for a couple of versions. True maybe not the fastest MacPro out there, but still serviceable enough.
I guess my main thought is regarding buying a 6 vs 12 core machine now ... if CS6 can actually leverage the cores even marginally better than now, the 12 core may prove the better machine for the next version of PS which is certainly within the life expectancy of the machine. I'm not even sure adobe can answer that question so maybe it isn't a factor in deciding.
So as a photographer if the only concern is max PS speed and no real taxing multi-tasking, the 3.33.ghz 6 core is the way to go. In my case, I spend less than 15% of time in PS, the remainder in C1 and LR, so the extra cores are proving quite valuable.