Given that i have the 5d1 and the fact that the focus system on the 5d2 was identicle, id say i would shoot a wedding with a 1dsm3 way before the 5dm2 for focus points alone. Saving time composing with center point and then re-composing would be worth it alone. On another note the 1dsm3 had a higher level of sharp detail hence why it is an incredible landscape camera.I agree with everything you said except, The 5D Mark II actually DID steal ground from the 1Ds Mark III. That is a well-known fact. I even bought the 5D Mark II instead back then, because it was cheaper and had the same resolution. Most of the pros I knew from 2008-current never owned a 1Ds3 simply because of the 5D Mark II.
A lot of 1DS3 were sold - but to a relatively small market niche
However the 5D2 filled another market and sold like hot cakes. Some pros bought it purely from a ROI point of view rather than for IQ.
Whilst the 5D2 had the same mps it lost out significantly on fps, colour rendition and AF.
There are a lot of 1DS3 on the used market now - showing that it did sell
I agree it sold. It was too specific though, whereas the 5D2 was well rounded. The problem with the 1Ds3 was it was much harder to shoot weddings thoroughly vs. the 5D2. Almost every wedding photographer I knew 2008-current got the 5D2 and passed on the 1Ds3. However, I feel that the 1Ds3 is the ultimate landscape camera. Ultimate.
You'd shoot a wedding with a 1Ds3? Haha, ok, good luck. Better bring a ton of portable lighting.