It has turned out to be a pretty good rule of thumb over the decades.

Isn't that still true for high perf chips? We don't have ways to use all those transistors so we make larger and larger caches.

Max-performance chips even introduce dead dummy transistors ("dark silicon") to provide a bit of heat sinking capability. Having transistors that are sometimes-but-rarely useful is no problem whatsoever for modern processes.

AFAIK the dark silicon term is specifically those transistors not always powered on. Doping the Si substrate to turn it into transistors is not going to change the heat profile, so I don't think dummy transistors are added on purpose for heat management. Happy to be proven wrong though.

My understanding is that pretty much every possible combination of these things is found somewhere in a modern chip. There are dummy transistors, dark transistors, slow transistors... everything. Somewhere.