The law of diminishing returns is borrowed from economics. The idea, when applied as a mental model, is simply that if you keep doing the same things, no matter how good or bad your approach overall, the returns will diminish. Fo example, if you keep tweaking a design, you’ll keep tweaking it for ages. After a while, it stops improving per se. The same goes for when you’re stuck in your ways. If you don’t try new ideas, your results decrease.