Canon will do it because it sells. They have now seen that there are enough who are trained to believe that more MP are better and will put their money down for bragging rights.
Its certainly possible to produce a 50mp sensor, probably a lot more, its a trade between file size, image quality, ISO response, readout speed, and frames per second capability.
The D800 is a reasonable compromise, you will need more memory and file storage space, and be willing to have editing go a lot slower (Its not linear with file size). I expect we will see something similar. Perhaps a 5Ds?