If people who design these things were playing the game they would never put a legendary with low % drop chance on a first boss. Instead would do higher drop rate on last boss.
Every Rogue I know has farmed their brains out to get that legendary in premades and also in lfg.
Don't assume it's great fun for those Rogue players either. It's a competitive game and the main tool of your preferred class is locked behind a stupid grind. You do it over and over in various configurations.
I don't excuse the quit after first boss, but I understand it and you should be blaming Blizzard for designing the game this way.