Let's say Canon releases the new 70D with a next-generation crop sensor... Now let's say this new sensor has noise performance as good as the 5D Mark II. (A stretch, but I think it's possible)
A crop camera with better ISO performance than the same manufacturer's previous full frame? Show where this has ever happened in history.
You're right, it's never happened in history. But maybe it will soon. (Yes, that's a big maybe!).
I'd guess that the noise performance of the new Rebel SL1 can't be too much worse than the original 5D's. I think the 7D Mark II has a good chance of having excellent noise performance. It will never beat the latest full frame bodies, but maybe it can give the previous generation of FF cameras a run for their money.
And what about using Sigma's f/1.8 zoom on a full frame camera? Sure, it's not designed
for FF, but from the Korean site with the samples it appears to mount and work properly on a 5D Mark II (though with heavy vignetting at all FLs below 28mm). If you're willing to lose resolution by cropping out the vignetting, then you actually can
achieve FF-level noise performance with this lens, and still shoot at an f/1.8 exposure. (AND get the full f/1.8 depth of field as well!)
I'm not trying to say that crop will be better than FF -- it never will be. I just want to clarify for the people who may not know or understand the facts: at the same ISO/ shutter speed, an f/1.8 crop exposure is always brighter than an f/2.8 FF exposure.
If crop performance can approach FF performance at only a fraction of the cost, then that's good news for photographers everywhere. That's why this new lens is a big deal and that's why I want to get one. (I don't even own any crop cameras right now!)