A big problem with increasing the clock speed is that your processor still needs to wait for the DRAM to respond. Modern processors are crazy smart about prefetching, caching, and running instructions in parallel, but at some point Memory bandwidth and latency begin to dominate over CPU clock speed. Many players have started looking at puting multiple dies on the same substrate with high-speed interconnects to reduce memory latency and improve throughput. Those improvements could help push the usefulness of higher clocks a bit further.