Could you elaborate on what you mean by "narrow"? My personal take is that it is actually quite a flexible system, although not very well supported due to the relatively small number of boons, etc. in published material. A "big book of alternate rewards" along the lines of Adventurers' Vault could have fixed this.
It's narrow because it tries to put everything in a little compartment to fit class advancement. You went and met this grandmaster of sneaking up on things. So he gives you this one niche edge after you jump through some hoops. It's too pat. It is very flexible within its narrow confines. I just find it's scope underwhelming.
Contrast this with something like the shaman training example in one of the versions of Rune Quest, where the character has to do all kinds of chores for the shaman in return for extended training. Now, the character can back out any time he wants, but the better extended stuff is coming later. Meanwhile, he is still a barbarian or whatever. Now, you can only go so far, not as far as RQ, because RQ is a skill-based system. But the principle is the same.
Me? I'd like to see the skill system be a major part that was split off almost entirely from class advancement, and treated mostly optional for those that want the traditional class-based game. Classes still give fixed bonuses to things, like rogues getting +3 to Sneak or such. I guess the current Backgrounds aren't too far from this. But then any advancement and broadening you do with skills becomes separate from your class/level and mostly about what you do in the game.
Same as with magic items, your opportunity for appropriate challenges to advance is somewhat limited by your level. You can't get a vorpal blade in most cases unless you handle some tough stuff? Well, you can't usually advance your diplomatic abilities unless you can talk to some hard cases. But it is possible outside of sheer levels, or with the right help, and so forth.
Or another way to think about it is keeping the traditional class/level part of the system simple while allowing for more richness to be layered onto the game optionally outside the classes and levels. It's possible, for another example, to have richer social system(s) if they aren't confined to a few feats, skills, and Cha mods.