I'd add that the 1DX seems more obviously focused on those sports, jurno, wildlife users than the 1Ds Mk3 was. The latters problem was I'd say that because it was also partly aimed at the studio/landscape market alot of those users didnt need its extra features so just bought a 5D mk2 instead.
The 1d4 was the body for sports and wildlife shooter
The 1Ds3 was for studio and landscape as you say. The 1Ds3 has a lot of extra features over and above the 5D2. The headline feature was of course the AF with everyone latching on to the number of AF points etc.
However there are a lot of little features on the 1Ds3 that dont get into the reviews - such as the ability to select up to 8 af points for metering, metering on the af point rather than the centre point and so on.
Other minor things for landscapers might be the 7 shot bracketing
Plus of course the standard series 1 benefites such as weatherpoofing, dual cards etc
The 1Ds Mk3 did clearly have alot of features that werent obviously aimed at the landscape/studio market though such as the relatively high FPS and advanced AF. Seemed to me like it was partly aimed at the jurno/sport/wildlife market aswell for those shooters who needed the benefits of a larger sensor at the expense of some FPS.
The 1DX seems much more focused on its target market so its likely that more of that market will be less willing to give up its features for a cheaper 5Dmk3/X.