That's a strawman. 6850 wasn't a true successor to 5850, was it? It was a cut-down 2/3 successor that still managed to only fall ~ 7% short. We all know that 6850 was more than a theoretical 6750, but less than a true successor to 5850. They probably should have called it a 6830 and called the current 6870 a 6850, instead.
I'm interested to see if with 28nm we'll actually get some new true midrange cards, like a full lineup of 77x0 and 78x0.
They've never managed to make such a large jump in the past, what makes you think they'll do it this time? 3870x2>4870, 4870x2>5870, 5970>6970, and TSMC is experiencing (again) major issues in their node jump. If 7970 is within 10% of 6990 then I'll be impressed, especially if it comes out several months before Nvidia's next offering.