Part of what the lower pixel density brings to the table:
The low-light capability of the EOS-1D X is evident in its incredible ISO range and ability to photograph in extremely low-light conditions. Adjustable from ISO 100 to 51,200 within its standard range, the new model offers a low ISO 50 setting for studio and landscape photography and two high settings of 102,400 at H1 and 204,800 at H2, ideal for law enforcement, government or forensic field applications.
And, they note that the noise level will be lower than any current sensor. That comes from the larger pixels/lower pixel density.
A lot of us back before the 1D3 and 5D2 were released were hoping for an uber-clean ff sensor with maybe 16MPs, and were a bit sad when the 21MP sensors came out. So I don't see this as a setback but more of a WIN for IQ while sensor technology catches up to enable mega-megapixels...