The problem with scaling Kabini up is that Jaguar is probably never going to run at 3.5+Ghz with 8 cores and stay in reasonable power envelope(say 65-95W). On the other hand 28nm Piledriver could do this without much problems. This is a pipeline design tradeoff. But since the IPC is always improving bit by bit with every generation of "big core" such as Bulldozer,we have come to a point when we will have SR core ,a high clocking machine, which also has (relatively) high performance
I agree with that, but once you look at AMD product mix you'll see that 65W-95W are a shrinking part of the market. AMD biggest market is mobile and there Brazos is king. With 65W and Bulldozer performance levels you are pretty much limited to very big notebooks, which haven't big margins, and above that it's a no-go, you can only go with desktops there.
On desktops, situation is a bit better for power hogs, but not that much better. Over 60% of AMD sales are comprised of 65W and smaller TDP chips, and Brazos is on its way to becoming the most important, or the second most important desktop product out there. Brazos is on the verge of outselling Trinity.
In the end, regardless of my opinion about Brazos/Kabini performance levels, I see in them a much better marketing potential than the entire big core line up.
Brazos and Kabini are the perfect products for AMD. cheap, small, efficient, and capable to offer competition to Intel but on its own terms. It is where they can offer a significant trade off, less performance for a significant smaller price, forcing Intel to eat into margins to field Celeron IVB/Haswell there, at least until 2014 when the "monts" arrive.
OTOH big core will be troubled when Haswell arrives. If Haswell does not bridge the IGP graphics it will be close to, it also brings IPC and power consumption improvements that will put pressure on AMD prices. There isn't significant trade off there, with big core their offer competition to Intel on Intel terms, and they are too far behind the technology curve for that.
As for 28nm, I'm kinda skeptical about it. It's a half node shrink, which means nothing too significant to power consumption and die size. Also it is a SOI-less node, which means more leackage. I don't see huge clock increases here.