I can get the thinking behind it, but the simple solution, namely tweaking your screen, just doesn't do anyone any good.
I'm sure most PC gamers out there don't appreciate having to adjust monitor brightness every time they play a different game. To me though this issue is a bit more unique. I happen to own an early, modded GSync screen. It has brightness and gamma settings, but due to the nature of these early (but professionally done & sanctioned by Nvidia) hardware mods, they are locked into 'pretty darned bright'. This has never been an issue with any game for me, it has always been possible to alter an ingame setting or edit an .ini file.
In an attempt to do something about this for Fallout 4, I have tried to add the old fGamma line to Fallout4Prefs.ini, in the Documents folder. I tried it at 1.400, which is what I have Skyrim at. If it had worked the same, it would have made the game noticeably darker. Unfortunately it doesn't appear do anything at this time.
Brightnes and gamma are key, basic features in any game. I can understand leaving out some kind of new advanced shader tech or lighting model, but not this. I hope it will be reincluded in an upcoming patch.