Supposedly the lifespan of the LED-backlit screens is extremely good and you really shouldn't have any meaningful reduction in brightness no matter how long the computer lasts. Older machines used a different backlighting technology that reportedly did dim over the years noticeably, and the brighter you ran them the faster this would happen. I never personally experienced this, but that's the theory anyway. I've certainly noticed this with projector bulbs, those things really dim over their lifespan.
Ultimately if you are working a lot with images and media, I don't think you'll really have the option to keep your screen brightness very low. Try editing your photos with the screen at any setting that really saves much power. It just doesn't work, at least not for me.
