Kent Beck argues YAGNI is fundamentally about timing, not code cost savings. Building speculative structure too early commits to guesses that usually prove wrong, creating wasted effort when requirements shift.

The core insight: two bills arrive from premature structure. First, reduced optionality—you foreclose better future options by building before knowing the real requirements. Second, Net Present Value costs—you sequence expenses ahead of returns, paying interest on capital that could earn returns elsewhere.

AI code generation changes nothing. When genies write speculative frameworks instantly, you still pay both bills plus lose comprehension since you didn't write it yourself. Perfect foresight doesn't save you from the timing penalty.

YAGNI was never about thrift. It's price theory wearing programmer slang. Build when you need it, not when code is cheap.