Clock speed != single-threaded performance. Clock speeds plateaued a long time ago, single threaded performance is still improving exponentially (by being able to execute multiple instructions in an instruction stream in parallel, as well as execute the same instructions in less clock cycles), though the exponent approximately halved around 2004 (if the trend had continued we would be at about a 100-500x improvement by now).
Hard to say it's still "exponential"...what do you think the current constant doubling period is now?
Here's the single thread raw data from that repo. If you take into account clock speed increase (which, as you agree, have plateaued) we're looking at maybe a 2x increase in instructions per clock for conventional int (not vectorized) workloads.
Is there even another 2x IPC increase possible? At any time scale?
If the trend had continued we would have 1500Ghz cores by now.