Complexity can result from features which pay for themselves. Complexity is not automatically always wrong.
At scale, complexity solves problems that you might not otherwise realize exist. Some of these problems are intrinsically complex, they can't be solved with a simpler solution. Not without "simple" organically growing into a much worse mess than the dominant complex solution that was designed by acknowledging complexity from the start.
But what I want to respond to is your trying to explain that "KISS is always the right approach". Rules of thumb like these are cheap.
They cause people to apply the same patterns regardless of the situation. Dispensing folk wisdom indiscriminately only serves to stop all thought and analysis.
Zealous rule-based engineering is the deathrattle of good systems design, and it makes me ill at ease.