I think the argument is that they've crammed way too many concepts into the Fighter class. If the Wizard, Cleric, Druid, Sorcerer and Warlock can all be separate class, there is no reason that every single 'guy with a sword' trope has to fit neatly into Fighter (or Fighter, Rogue and Barbarian). I've come up with 8 subclasses for my Warlord and I'm an amateur designer. Somehow the former trained soldier and the Chosen One who just picked up a sword are the same thing at level 1? Why should that be the case? I think the Fighter would benefit from being defined by its more formal training (compared to a wilder Barbarian and an underhanded Rogue) and another class could pick up the slack of the nobody.