Isnt it that at some point the GHz just aren’t useful anymore or rather not physically possible. I think they abandoned it for a good reason.

I think there are two parts to this. There are factors beyond clock rate; clock rate alone doesn’t give a full picture. Going from say 166 mHz to 1 gHz brings radical performance improvements without too many drawbacks, once you go above 3-4 gHz, the marginal increase in clock rates becomes increasingly expensive in terms of heat management.

Watch out for your prefixes, 166 mHz would be one operation every 6 seconds.
I don’t think there ever has been a CPU that slow ;-)

(small letter “g” doesn’t exist as a prefix, but could be confused as the unit gram-Hertz)