Yes, I was just kidding. But in all seriousness seeing that they did it with the rMB it only makes sense if it trickles down to the other consumer products that they make. However, I do think a 'Space grey' iMac could look pretty cool.
It would definitely look nice, and given that the iMac isn't really a mobile computer, that finish should last a long time. I'm always worried about how different colors may show wear and tear on a laptop. The iMac has no such issues.
Yes, I don't see why it wouldn't last as long as the lifespan of the computer, unless it is dropped or something like that. I do not own a rMB but they should be a good indicator for how well the different colors are holding up.
I ran games just fine at 5k. Now they weren't top of the line GTA 8 or whatever it is you people be playing these days but they were games nonetheless.