What do you see as disadvantages?A similar problem happened to the 1Ds3. When the 5D2 came out the extras provided by the 1Ds3 weren't really worth it for many photographers.
The problem the 1Ds mk3 had though is that alot of its potential market was studio/landscape users and to them its advanatges in AF and FPS over the 5D mk2 werent really very relivant. The 1DX on the other hand seems that dispite the hype its marketed much more towards the 1D userbase so while a say 1DX sensor, 7D level AF, 6 FPS 5D mk3 might be an alternative it would be one with alot more disadvanatges.
Well in that case for a sports/jurno/wildlife shooter they would be lower FPS, worse AF, no protrait grip, worse build and a bunch of other features.