Godzfirefly:
My personal opinion (and I'll emphasize that it's ONLY my opinion) is that E6 is a limited attempt to fix the faults of the deeply flawed 3.5/Pathfinder system.
It is less of a flaw, and more of being diffierent from popular desire.
The core d20 design is centered on naturalistic balance rather than gamist balance. Primarily because it was dssigned to be played similarly to how Gygax and Arneson played, which is quite different from the popular use of the rules.
A major part of that touches on your worries about complexity. It is a sad fact that many, if not most, players see rules as constraints, and while houserules are generally accepted (if minor. Major houserules are avoided like a plague.), having more rules generally makes players less accepting of twisting those rules during play, even when the rules are spevifically designed to be twisted, such as with d20.
D20 was not designed for rules to be seen as constraints, nor was it designed for rules to be
what players can do, instead, it was designed for rules to be a supporting framework, for the rules to be
how players can do something. Following the intended use, you could have a table of players who never see character sheets, rules, or dice, and have them play almost freeform style telling the gm what they do, and the gm could any action (that makes sense within the world milieu. I.E. no turning off gravity in a contemporary setting.), run it through the system and get reasonable and sensible results.
Strangely, even though the rules were designed that way, few players want to play that way.
That is one reason splats tend to get power creep. The designers of splats are usually not those who designed d20, and so they design for what players want, which is options and to feel powerful.
Additionally, is the issue of advancement, which is d20's real flaw in this case. D20 has set up a case where advancement grows in both power and versatility combined (and not much versatility gain, unless a caster). That is an issue mostly because players like advancement.
To use an example, Aragorn from The Lord of the Rings, is basically a lvl 5 from start to finish. Trying to get players to play through an epic saga of that scale and scope without any advancement would likely result in a new gm and a new game.
You can't just blame the system. The player's expectations and
how they use the system is as much a factor as the system's design.