Like let's say you can't beat a boss and you go get better gear, or you get group members with better itemlevel, so that it's easier to kill the boss, you do more damage, the boss dies faster, the damage the boss is doing is easier to survive... isn't that like lowering the difficulty of a fight straight away? Like if you were playing a single player game and you were at a boss fight and you just lowered the difficulty, so the health bar of the boss goes down quicker and he doesn't hit as hard. That's what it's like to outgear a fight.
But in that sense, every gear upgrade lowers the difficulty of a fight. So why try to get better gear? Why not just do the fight at a lower difficulty? In the end getting better gear will have the same effect as doing the boss on a lower difficulty will have.