I would say because the technology to install 2 cards are already here, and that only enthusiasts are crazy enough to install 2 GCs, SLI (and all spinoffs) will be here to stay. Of course, there's nothing to stop GC makers from fitting in more cores on a single card.
Again, I think multicore + SLI is only for people with watercooling. Just too much heat into the case. As it is, temperatures in my casing is more than just a couple of celcius higher than ambient, and thats with alot of fans. Quiet ones, but there's a total of 4 for the casing alone (including the noisiest 120mm exhaust pushing >60cfm). And I'm not even running SLI.