Transistors are being made smaller and leakage current is going up. DARK SILICON: we can no longer turn on all of the chip: perhaps one tenth maximum for today's 22~nm chips. Even less in the future.
Slow, bulky power transistors will turn thousands of power islands on and off under automated or manual control.
Incresing use of high-level synthesis (HLS) of standard software kernels into application-specific hardware coprocessors and put them on the chip just in case. They have negligable cost if not turned on?