Tom, I think your comment speaks to the fact that the current crop of cameras, especially the D90, were real quantam levels in photography. On another thread, I mentioned that my 90 was the first piece of electronic gadgetry that I haven't longed to update after a year of ownership! My latest iMac is also falling into that category also.
I'm not so sure that the D400/8000 will be an equal jump, but who knows? For most users in the prosumer market, I think the 12-15 mp sensor is more than adequate. Improvements in ISO and AF abilities will set the pace, but honestly, they're so darn good right now, at a darn good price, I think it will take more than most would be willing to pay (and nikon marketing willing to set prices at) to set a new standard.
Just idle speculation based on my narrow experience band... And wouldn't you just love to be a fly on the wall at an R&D facility at Nikon, Apple, or Intel, and see what's on the chalkboards?
Dave F Franklin, Pa
"Always do right. You'll gratify some, and astound the rest." Mark Twain