Will the brightness of the headlight bulb decrease over time or do they just burn out before dimming?
The lumen's will decrease over time, also it will color shift. Thats why you should always replace both bulbs. I bought some Philip's crystal bulbs 2 years ago for my van and I need to change them now. My van has poor poor lighting and the bulbs did help. It's also preference.
I wouldn't spend the money until they quit on you, but that's just me.
I hear ya and hate to BUT I spend a lot of time outdoors on/at my ranch and want them as bright as possible without that stupid (IMO) blue color. Just seems the light is more "amber" than they were when I bought the truck last year.