It doesn't destroy the challenge unless you want it to destroy the challenge. If you don't want to be lead through the dungeon by someone level 50, don't get someone level 50 to join you, form a party of level 25's instead.

If someone ones wants to be hand-lead through the dungeon, what difference does it make to you?
Because people (including me) will always take the easier route in a competitive environment like a MMO. There is no "free choice" when　EXP/hour or loot is involved. Your argument sounds logical, yet defies human nature. If everybody was offered artifact weapons at the beginning of the game - who would refuse? Would it make their gaming experience more fun? No.

SE has to take into consideration human nature, but that's one of the many things they fail at.