If you want hardcopy, then the pixel-for-dpi ratio must be good enough...thus the R-D1 2000 x 3000 sensor is good enough for an ~8 x 10 if a 300dpi printer is used.
The 12Mp APS-C sensor used in the X100 would have to be ~3000 x 4000 @ 6u, and a good 8 x 10 or even an 11 x 14 is possible.
That kind of pixel count is also more than good enough for display on a 1080p (1920 x 1080) LCD/Plasma penal. I suspect that is why the camera has both mini-USB and HDMI ports built-in [under the right-side hatch].
My home TV has long been my revolving gallery.
In theory, I agree. But as Yogi Berra said, In theory, theory and practice are the same. In practice, they aren't.
A few things that modify this simple calculus:
• To a first approximation, no one uses a tripod and good technique to reduce vibration to negligible levels. The spatial frequencies that are actually captured in handheld photography seldom exceed 1000 lp/image height. More pixels won't help.
• All but the very best (read: expensive) lenses are already Nyquist oversampled on a 12 megapixel sensor. Again, more pixels won't help.
Sure, some pros and crazy amateurs use tripods and have immaculate technique, and they are of course overrepresented in forums like this. But the vast majority of people don't own a tripod, and the vast majority of people who own tripods seldom or never use them.
• Additionally, as print sizes increase, so (usually) does viewing distance, and the 300 dpi requirement can, except for very high-quality (exhibition) prints, be relaxed.
For most people, including a majority of "amateur photographers" and a pretty large number of pros, the already available APS-C sensors provide more IQ than needed. As these sensors are continuing to improve, the need for FF equipment is, for most people, less and less pressing.
I acknowledge that there are a not-insignificant number of people who can benefit from FF, and even more so that there are many gearheads who just like cool stuff.
But I also think that if you put a 7D or a D7000 or even an E-PL1 in the hands of most people, they will already get far better IQ than they will be able to use, and they'll be able to make bigger prints than you think they will. If these prints aren't good enough, it generally won't be because of sensor limitations, but because of camera movement.
• I haven't even mentioned focus accuracy. One of the tremendous benefits of smaller sensors (APS-C, 4/3) is that DOF is increased, relaxing the requirement for perfect focus accuracy.