What you describe is practically what many OEMs do. they update the driver and emitter, and sell the entire light product as a whole new world.
Designing, engineering, testing, packaging, distributing, and selling an upgrade pill would be as much work as selling the entire flashlight for a lot more money. In fact, it would be way more complicated explaining to wholesalers and end customers what a pill is and how someone would buy and use one.
The P60 design was a successful attempt at selling upgrades of flashlights; if you look at the flashlight world today you can see the concept didn’t survive.
Many OEMs do it this way, a car manufacturer prefers to sell an entire oil change to you - and not the filter, o ring, copper washer, oil and other parts as a kit. In fact, they hate it if you buy oil off of Amazon and bring it to the service.
Why does Apple solder in almost all computer components now? Thinner devices, less product complexity, streamlined production processes, etc… but also to avoid prosumer DIY upgrades.