I keep seeing people using the word "usage" to refer the percentage from task manager, but that really isn't usage, it is "unavailability."
People should be familiar to prime95, and although tests are all 100% CPU, some does make CPU hotter than others. When it is 100%, it doesn't mean it is at 100% load, but it is 100% unavailable.
The reason why newer chipset/cpu can acquire better performance with the same clock speed is due to the bottleneck where the core is unavailable, but isn't doing anything but waiting. Hyperthreading is introduced to utilize CPU better.
Back to the topic, as I have mentioned, since newer chipset reduces the idle time of CPU while it is unavailable, performance increase even though it runs at the same clock speed as older chips, so it isn't just about the clock speed. You can OC a C2Q and it cannot beat i7, and the difference gets bigger as clock speed increases. Besides that, newer chips can often acquire higher clock speed, making it even more attractive compare to old chips, not to mention the newer chipsets which supports more goodies.
The only problem is, newer chipsets don't support older CPU, and newer CPU don't support older chipsets, meaning that consumers will need to upgrade both. Luckily, newer chipsets still support DDR3.