This creates the “illusion” of an ever-progressive complexity. But it isn’t something that can be quantified. If it could be quantified, we would be able to answer the question:
If it could ever happen that a fish population becomes a land-based tetrapod, is that a gain in complexity or a loss of complexity?
The tetrapod has lost its gills, but gained lungs. It has lost the ability to swim, but the tetrapod has the ability to run.