Legendaries are not required to kill any boss or to progress in the game in any way. Do they help? Yes. Are they mandatory? No.
It's completely a player-generated problem.
The only way Blizzard could go about it would be to either nerf them to a point where they're barely an upgrade at all, make them so rare that next to no one had them, or remove them completely. Player-mentality is the problem, and the reason it's hard to make up good things in WoW.