As for many here, 18MP is beyond me.
I don't believe that Canon is going to introduce another 3rd 1-series cameras, that would be costly to manage, rather I believe the opposite meaning merge (not in the current case though).
Now, one proposed explanation was more than 10fps for the new cam, meaning Canon is not that bad in data processing capabilities. However, 18MP is completely 180-degree turn by Canon, a slap in the face for former users. A new religion must be introduced by marketing.
Trying to rationalize the global picture, less megapixels could explain a new 30+ (only!) 1Ds, which would have been skipped by potential buyers if the 1D was having something like 27MP (upscaled 1D MK4), fullfilling majority of users needs. The key here is to keep enough difference in MP to sell both.
For all high ISO fans - it is not the MP you are after but rather the sensor size, 30MP wouldn't make it worse than 18MP in print using the same technology, rather the opposite, more sensels enable better IQ.