quantum mechanical limits that can never be overcome, by any technology
That's a confident Lord Kelvin-like statement about physics, and should be treated as a failure of imagination. Physicists agree that they still do not understand the elusive boundary between quantum and classical, so talking about some unmovable limit in that area is pretty silly. Then again, Richard Carrier is a historian, not a physicist.
For example, if you subscribe to the MWI model, gaining access to the googolplex of the worlds created every femtosecond and harnessing their computational resources would effectively remove anything resembling a computational speed limit.
Another example: we might discover that around the Planck scale the world consists of something like unparticles, and their scale-invariance would allow us to miniaturize without a bound.
Given that it took me two minutes to come up with two (admittedly far-fetched) examples of how the "no further advances will be possible in terms of Moore's Law" statement could be wrong, and I am not even an expert in the area, I would discount all his predictions as too poorly thought through to care about, until and unless proven otherwise.
For example, if you subscribe to the MWI model, gaining access to the googolplex of the worlds created every femtosecond and harnessing their computational resources would effectively remove anything resembling a computational speed limit.
So, you can think of this as what quantum computers do, and there's still a pretty normal speed limit. Because all (traditional) interpretations of quantum mechanics run off the exact same math, a good test to apply in these cases is that if it only seems to work in one interpretation, you've probably made a mistake.
A...
Recently I stumbled upon Richard Carrier's essay "Are We Doomed" (June 5, 2009), when asked to comment about the Singularity, said the following:
What do you think?