I have a feeling Slashfan actually has the correct definition of complexity, as opposed to your definition.
It's not good to make judgements when you have no idea what you're talking about

Please, tell me what you think I listen to. ? Or my experience level in music?
Basic? You sir need to hear more of their music....some of that stuff is extremely complex.
Look, don't think that I'm down playing Maiden's ridiculous success in any way, but they just do not stand out musically when compared to others outside of their genre.
It's not often you will find drummer who plays his part more complex than the rest of the band's, which can tell you the overall creativity and range the band will utilize. Not to mention that would be musically incorrect.
Compare Nicko McBrain to someone like Benny Greb, Navene Koperweis or David Garibaldi (to name a few) and tell me who you think is more musically educated.
Take two common bands like Animals as Leaders or Tower of Power for example; not only are both in completely different genres but most if not all of what they have released is quite a large margin more creative and dynamic than that of Iron Maiden.
Also, to compare record sales and popularity to raw talent would just be stupid. Sadly, musicians and bands in genres such as Jazz Fusion, Progressive Instrumental, Funk or Big Band will never ever reach the levels of popularity Maiden, Metallica or Guns and Roses did simply because people don't listen to it/won't understand it.
To put it simply, there are two kinds of musicians. Ones who know what they're playing and ones who don't. You can easily categorize Maiden as a jam band, rather than one with the proper education and background.