A 1Ds "downgrade" to the 5D format - do you really need the portrait handgrip on a studio oriented camera? - might appear sooner and be better value.
Isn't the 5D II basically a "downgraded" 1DS III?
No, it's not. It's basically 1Ds3 sensor put into the original 5D body, plus video.
Mightn't the 5D III follow the 1DS IV in a similar fashion?
You mean, like put the great sensor to the inferior-AF-crippled* body again? I wouldn't be surprised.
* 10 years after EOS-3, 5D2 AF was nothing but intentional cripple to protect the 1D line. Canon just tried to pretend that there was no advance in the AF technology for the prosumers class. Period.
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Cheers,
Martin