The Casio F-91W costs $30 and runs on the exact same chip from 1989. Same case. Same plastic strap. Same module 593. Casio has sold over 100 million of them. The product strategy here might be the most counterintuitive lesson in consumer electronics. The bull case for killing this watch and building something new looks overwhelming. Apple Watch does health monitoring, GPS, cellular, fall detection, notifications, apps. It's a computer on your wrist. The F-91W tells time, has an alarm, and runs…