There's arguments to made for the economy and exp/leveling for fatigue as well. It is not just in place for the sake of botters. I just wanted to say that.
Now, I, personally, am fine with fatigue, both as a player and as more of designer (which I am not, but just thinking from that perspective). As a player, it does suck when first making a character, yes, as you just skyrocket to level 20-22 and then you just hit a sudden stop. Beyond this, however, I've found it helps me trudge through and get those few extra runs done to max out my fatigue. It adds something to think about (if I choose to think about it) and is just another bar that it feels good to max out.
From a design perspective you can control how long it takes a player to get to max level if they play as efficiently as possible, so you can make it take something that's not super quick but not an extreme drag. It keeps efficiency more at the forefront of players' minds both because there's a bar there letting you know you can be more efficient, but because maximum efficiency is actually attainable now. In other MMOs, maximum efficiency is literally grinding 24/7. Minus swapping out and botting, it's unattainable. With fatigue, it is attainable. For some, it's not as fun to be 100% efficient, but those players (me included) will probably just play inefficiently, be it slightly or extremely. This does bring up the issue that they could easily burn themselves out doing this, however, and I don't really know what to say to this personally.
I would also like to mention that maybe fatigue is a bit too limited currently, or maybe should curve dependant on level by some means (be it actually or indirectly via early quests rewarding extra fatigue consumables, etc). It's not perfect, don't expect it to be. I play the game for the fan-fucking-tastic combat and the just gorgeous class diversity. No other MMO, no other game gives me this experience, so fatigue vs blitz has never been at the forfront of my mind. For me, I will admit, I remember blitz under Nexon and the hell that leveling was back then, so my views are somewhat skewed.