> Nevertheless, an optimum seems to have been reached around 20 years ago, and since then more often than not I see only worse interfaces that make harder

A consequence of having "UI designers" paid on salary instead of individual contract jobs that expire when the specific fix is complete. In order to preserve their continuing salary, the UI designers have to continue making changes for changes sake (so that the accounting dept. does not begin asking: "why are we paying salary for all these UI designers if they are not creating any output"). So combining reaching an optimum 20 years ago with the fact that the UI designers must make changes for the sake of change, results in the changes being sub-optimal.