> the (real) purpose of a system is what it does

I agree with the rest of what you wrote but the quote is an overly cynical tired cliche when applied in a blanket manner. There are specific situations involving bad faith actors where it is directly relevant, and there are also times where it can be a useful observation about the impact of perverse incentives that build on top of unintended consequences.

But the way you're using it there it's no better than other politically charged nonsensical slogans.