It's a little more formalized and generalized observation: https://en.m.wikipedia.org/wiki/Amdahl%27s_law#Derivation

But yeah! There's a lot of common sense that, with some mathematical formalization, yields useful and extensible laws.

Amdahl's has been robust because it opined on the 60s supercomputer coprocessor architectures, and then in more recent decades with consumer multicore chips.

Laws aren't famous because they're insightful -- they're famous because they're useful.

These laws make more sense if one reads them as "heuristics", which is what they are.