It's important to remember that just because clock speed isn't increasing at a large rate anymore doesn't mean the micro-architectures are standing still, even at a single-core level. Some of the improvements in the core microarchitecture over say... the P4 microarchitecture are welcome and should yield performance improvements.
That said, I would agree in general that single-core performance is not indeed climbing as much as it used to be, but to say it's completely standing still probably goes too far.
Even the clock speeds on consumer chips may still have room to improve. IBM was surprised to see everyone stop at 3Ghz which is why they turned that crank one last time to hit 5Ghz in POWER. They mostly did it for marketing purposes, but in terms of long-term trends, they seem to feel that that's the place clockspeeds top out. Not where we are now.
That said, none of this refutes your point really, but perhaps provides some different color to it.