My iPhone 6 screen seems to get getting dimmer as time goes on, I will crank it all the way up and it's about the same level of brightness as my work SE (new this past year) set at 50% (which can be seen from space at 100%)

Do these screens burn out? It's over 3 years old now, but I'm not replacing until at least this summer or fall, otherwise it works perfectly.

Is the front clean? Maybe some buildup causing the auto-dim to mistake it for a dark environment. Also try turning off the auto-dim in the brightness settings?

No, it's called 'getting older'. That'll be $50. Pay the receptionist

I've been using "Night Shift" more often and have it set for 12 hours, 7-to-7. But sometimes forget when I'm in a lit space, and the screen seems dim.

Apple technical support is, amazingly, free in any store. They can tell you what is up. Sometimes I know more then they do but usually they either confirm what I cannot figure out or have the answer.

if you google 'iphone 6 screen dark' or dim in place of dark - you'll find a bunch of tips for slapping your brightness settings around.

No...check your brightness settings