Simplicity as a precondition for verification
Economy of mechanism holds that a design should be kept as simple as its actual requirements allow, because simplicity is what makes a system's correctness genuinely verifiable — a reviewer can reason completely about a small mechanism with a limited number of states and paths, in a way that becomes practically infeasible once a system accumulates enough incidental complexity that no single person can hold the whole thing in mind at once.
This isn't an aesthetic argument for minimalism. It's a claim about where unexamined gaps tend to live: not usually in the parts of a system that got careful, deliberate attention, but in the corners that grew complex enough that nobody actually traced every path through them.
Applying it to authorization specifically
For an authorization system, this favors a small number of clear, uniformly applied rules over a sprawling set of special cases, exceptions, and one-off overrides accumulated over time. Special cases are sometimes genuinely warranted — but each one adds a distinct path that has to be independently understood, tested, and kept correct as the rest of the system continues to change around it, and the cost of that compounds with every addition.