I think newer lithium batteries have longer lifetimes now. Better battery design and manufacturing methods?
My old Dinotte lithium battery started getting short run times after just 2 or 3 years. But my current Cygolite blinky headlight has been holding good run times for 3 or 4 years. I run it on single flash on every ride, and charge it every two or three rides.

Same with my Makita 18v drill batteries. These are still going great after four years. (it's way less usage than a bike light, but my older nickel metal drill was useless after just 2 years.)