nVidia is far from the only chip designer looking into stacked RAM, they're merely the ones with the best press releases about it because nVidia knows a big part of their consumers are enthusiasts who get excited about hearing about what they're working on. Rest assured that if AMD can stick around long enough they will have stacked RAM within a generation of nVidia.
This.
From 2006, let's see what Intel did:
2006 => Conroe 65nm
2007 => Penryn 45nm
2008 => Nehalen 45nm
2010 => Westmere 32nm
2011 => Sandy Bridge 32nm
2012 => Ivy Bridge 22nm
2013 => Haswell,
So in a span of 7 years, Intel developed 7 generations of their architecture.
In the mean time, AMD got:
2007 - Barcelona
2008 - Deneb
2011 - Llano, Bulldozer
If we disregard the small tweaks included in Zosma, Thuban and Vishera, that's four generations since 2007.
While it is clear that AMD designs are subpar, the other part of the problem is in the foundry. AMD, and later GLF could not field node processes at the same pace as of Intel, so even if AMD designs weren't subpar as they are now, AMD still experience disadvantage to Intel because of both inferior electric properties of a chip and bigger die size.
AMD GPU division didn't have this kind of problem when dealing with Nvidia, because both were in the same node of the same foundry. They also had access to the same technology at the same time, so whatever differences between the two companies were due to design tradeoffs and software support.
The day AMD moves GPU production to GLF, this symmetry is broken. One new generation in Nvidia won't necessarily mean a new generation for AMD because GLF might not have the node ready. Same with features. NVDA might have a feature X in the node N because TSMC does support feature X, while GLF might not support feature X in the node N, only in node N + 1, further eroding AMD position.
Sure, it won't be as bad as it is in the competition against Intel, but GLF track record isn't anything inspiring and they aren't up to TSMC standards too. And given that AMD GPU business is operating with razor-thin operating margins, I cannot be anything but pessimistic about the future of AMD GPU business.