In addition, this is due to wire delay not being able to scale at the same rate, and power limitations due to leakage current.
It is interesting to note that single-core performance benefits are not completely overlooked. Different processors have varying single core performance: a quad core processor may have stronger single core performance than a 8-core processor, which may make it perform better in certain situations. For example, the gaming industry is slowly moving to supporting more and more cores, so right now, some games perform better with processors with stronger single cores. However as developers begin to efficiently utilize a larger number of cores, this may not be the case anymore.
Although the transistor density is still increasing, it can not double the performance of processor. The ILP have no space to improve when it reach about 8 instruction per clock(actually most of them are 4). The clock frequency cannot increase, since as you increase frequency, you will get non-liner increase of power. So it's now software developer's job to gain the performance.
I am curious about what performance will the processor get when there are more transistors. Maybe it can get larger cache? More complex logical performance? Or faster memory access? Also, more transistor means more power. Does it mean that in the future, the green line will stop to gain?